G'Day Davo,

To the best of my memory, the Brit's FN's/SLR's, used to be issued with a synthetic butt stock and forward hand guard.
Try one of the many US suppliers that advertise in "Shotgun News" magazine and On-Line(?).

Then all you have to contend with, is getting an import permit off the authorities for a "Prohibited Weapon" (or Parts, there-of) and the Australian Customs Service when it arrives!

Good Huntin!

Doh!
Homer