Its getting quite cold at night here in Brisbane too (though probably not comparable to you guys down south!)
On the very cold nights I dress Aidyn in a singlet, skivvy, pajamas, socks and his 1.0 tog grobag. On the nights that feel a few degrees warmer I will not put the singlet and socks on him.
We are pretty lucky that we can get away with using the 1.0 tog grobag... it means he can get heaps of use out of just the one grobag through Autumn, Winter and Spring!