After a pm from Biathlonman, I rechecked with Scouts Australia, and was advised there is in fact regulations covering the wearing of kilts in Australia.

Kilts maybe worn by Scouts, but not Joey Scouts or Cubs.
The wearing of the kilt has to be authorised by the branch president.
The Scouter must be Scottish or of Scottish descent.
There is no mention of any particular tartan that must be worn

Thankyou Bialthlonman for the information