It is approx 2cm long and was discovered in house after a bush walk & BBQ at Riverside Garden Reserve in Mandurah, Western Australia. It has 6 legs.
This is a Sawfly larvae, and is most likely of the species Perga dorsalis or Perga affinis. The larvae you depict appears to be quite young, but as it matures it'll gain a much darker color, and, depending on whether it's P. affinis vs. P. dorsalis, the abdomen will grow fairly long.
I imagine the developmental progress of your larvae is somewhere between the two following larvae.
A few more images: