BACKGROUND ART Security fences are commonly known barries to restrict or prevent the access or egress of unauthorised persons to or from a fenced area such as for the protection of property and belongings. Security fences are also used to confine persons such as inmates of a prison or detainees of a detention centre to a confine area. The effectiveness of prior art security fences is often limited to the height of the fence and whether the fence inclues other barries or obstacles such as barbed wire or razor wire to deter climbing over them. In many cases security fences with such barries can often be overcome by simply placing a heavy mat or similar material over the barbed wire or razor vire. In order to overcome this problem, prior art security fences often have a barrier of barbed wire at the top of the fence which is also angled away from the fence.

Commonly, the fence may in fact have two barries of barbed wire in a Y shaped configuration to make the fence more difficult to climb across. Even where the Y shaped barrier is used, determined climbers can still cross these barries by placing heavy mats over them.

Australian Patent No. 237170 describes a flexible guide post employing a coil spring fitted into a concrete footing. US Patent No. 4,605,204 describes a collapsible recreational fence employing a resilient plastics mesh with plastic posts. German Patent No. DE 3834817 describes a security fence with small springs allowing limited deflection of an upper part of the fence to activate a <BR> <BR> <BR> <BR> microswitch to detect intruders. UK Patent No. 1354057 describes a security fence where strain gauges detect interference with the fence. US Patent No.

1,336,680 describes a fence post used to allow passage of driftwood to prevent the fence being damaged during floods. US Patent No. 597,2625, describes a resiliently deflectable traffic bollard with rotatable collar.

While the above referenced patents describe various forms of post and fence deflections they are either expensive to manufacture or are unsuitable as part of a security fence of the chain wire type. The present invention provides a

security fence that is economical to construct as an alternative to existing security fences and provides a resiliently deflectable upper portion positioned above head height secured to a rigid lower portion to inhibit unauthorised persons from climbing over the fence.

OBJECT OF THE INVENTION It is therefore an object of the invention to overcome to some degree the problems associated with prior art security fences.

SUMMARY OF THE INVENTION In a preferred aspect the invention resides in a security fence having the improvement of fence posts comprising upper and lower portions; the portions connecte by a flexible member biased to keep the upper portion in one position relative to the lower portion, wherein in operation, force or weight applied to the upper portion causes the upper portion to collapse in the direction of the force or weight; the upper portion returning to its original position on removal of the force or weight.

Preferably there is provided a security fence comprising spaced posts extending above adult head height, each post having an upright rigid lower portion and a resiliently deflectable upper portion, a major portion of the length of said posts comprising the lower portion and the upper portion having a lower marginal section thereof above head height, barrier material interconnecting the lower portions of adjacent posts to prevent unauthorised access other than by passing over the security fence, barrier means interconnecting the upper portions of adjacent posts, the upper portions resiliently deflecting under load to inhibit unauthorised passage over the security fence. Preferably the upper portion is joined to the lower portion by a resilient connector. Preferably the resilient connector comprises a coil spring and coupling collars extending over marginal end portions of the coil spring and coupling opposite ends of the coil spring to the upper and lower post portions respectively.

Preferably the upper portions comprise Y-shaped terminal sections with barrier means extending between respective arms of said Y-shaped terminal portions of adjacent posts.

Preferably the upper portions are adapted to deflect through an angle of greater than 45°.

In another preferred aspect the invention resides in a security fence comprising:- at least two spaced upright posts; the posts having an upper portion and a lower portion; the upper and lower portions connecte by a flexible member biased to keep the upper portion in one position relative to the lower portion; a panel member spanning between the lower portions of the posts; a barrier member spanning between the upper portions of the posts, wherein in operation, force applied to the barrier member or the upper portion of the posts causes the barrier member and the upper portion to collapse in the direction of the force; the barrier member and the upper portions returning to their original position on removal of the force or weight.

Preferablythe posts are galvanised iron of circular or square cross sectional profile.

The upper portion is preferably shorter in length than the lower portion of the posts. The upper portion may be in vertical alignment with the lower portion or may be angled away from the lower portion. In the alternative, the upper portion may comprise of two or more sections of post, the sections splayed relative to each other in a V or Y shaped configuration.

In preference the flexible member which biases the upper portion to remain in one position relative to the lower portion of the posts is a coiled spring however other types of flexible members may be used.

Preferably the flexible member is connecte to the upper and lower portions by one-way security fasteners which require a special tool for their installation and removal. Other methods of connection such as welding the flexible member directly to the upper and lower portions may also be used.

Alternatively, if ordinary fasteners such as hexagonal nuts and bolts are used, the nuts maybe deformed or burred over to discourage removal.

Suitably the barrier member comprises one or more rows of barbed wire or razor wire.

Preferably the panel member is wire mesh however timber or other panel material may be used.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the present invention be more readily understood and be put into practical effect, reference will now be made to the accompanying drawings where:- Figure 1 is a front elevation of the security fence according to the invention; Figure 2 is a side elevation of the fence of Figure 1; Figure 3 is a side elevation showing the operation of the invention; Figure 4 is an exploded view of the flexible member according to the invention; and Figure 5 is a cross sectional view of the flexible member according to the invention.

DETAILED DESCRIPTION OF DRAWINGS Referring now to the accompanying illustrations and initially to Figure 1 there is illustrated a security fence 1 comprising a panel of wire mesh 2 supporte by posts 3,4 comprising lower portions 3b, 4b and upper portions 3a, 4a. The lower portion 3b, 4b and the upper portion 3a, 4a of the fence posts are connecte by a flexible member 5. The upper portions support a barrier of barbed wire 6. Also shown in this illustration is a representation of a man 7 of average height for height reference purposes.

Figure 2 shows a side elevation of the fence of Figure 1. In this embodiment the upper portion 3a of the fence post is angled away from the vertical and is connecte to the lower portion 3b of the fence post by the flexible member 5. Also shown for height reference purposes is the figure of a man 8.

Figure 3 shows the operation the flexible member 9 when the weight of the climber 10 causes the Y-shaped upper portion 11 to collapse in the direction of the climber 10.

Figure 4 shows an exploded view of the flexible members. The flexible member 5 comprises a coil spring 13 which is affixed to upper and lower portions of a fence post (not shown) by means of semi-circular collars 5a, 5b, 5c, 5d. The semi-circular collars have a cross-sectional profile 14 to accommodate the coils 15,16 of the spring 13 which are secured between the two semi-circular collars 5a, 5b and 5c, 5d by means of bolts 7 having nuts 8.

In this embodiment, ordinary nuts are shown which may be deformed or burred over to discourage removal. In the alternative, security nuts which require a special tool for their installation and removal may be used.

Figure 5 shows a cross-sectional view of the flexible member 5 of Figure 4 in the fully assemble position. In this view the coil spring 13 is securely held between the semi-circular collars 5a, 5b and 5c, 5d which have cross sectional profiles 14 to accommodate the coils of the spring 13.

Security nuts 8a are used to secure the bolts 7 with the collars 5a, 5b and 5c, 5d against the coil spring 13 and the upper 17 and lower 18 portion of a post.

Whilst the above has been given by way of illustrative example of the present invention many variations and modifications thereto will be apparent to those skilled in the art without departing from the broad ambit and scope of the invention as herein set forth in the appende claims.