The law in Aus is that they can't be turned around until 8kgs, and with most seats they can only stay rear-facing until 12 kgs. Rear facing is MUCH safer so I highly recommend you leave them that way until 12 kgs. This is your choice, but you really shouldn't turn before 8kgs, it is just not safe. In Sweden (I think, it's one of the scandanavian countries), they make bigger rear facing seats and by law all children travel that way until 4 yo.

We only turned DS2 around recently, he was nearly 18 months then.