As far as I have read they were not.
They were not available in numbers until late in 1942. I've seen photos of them up north in the defence of Australia (I don't know if they were with AIF or Militia units though) and have also read that they were issued to VDC units (volunteer defence corps) in NSW in 1943(?).
On another note the Mortar carriers probably did see combat although not with Australian forces.
Nice model kit too.