From memory, in Victoria it is 6 months or 8kg, whichever comes first... Mind you we decided to wait until after DS turned one (even though he reached the 8kg mark way before that). By that point he was walking well and I knew he had the strength in his spine iykwim? Personally I wonder if I should have kept him rear facing longer (he still hasn't reached 12 kg which is the limit for our seat), but at the same time I do find it so much easier having him face forwards. I think if you're wondering about safety etc and why you should wait longer than the minimum time/weight allowed then it's worth doing a search on youtube, there a couple of videos that show the impact a colission has on a little ones neck (they aren't fun to watch, but put things in perspective)...
It's recommended to leave them as long as they are under the weight requirements for rearward facing. You can get car seats that go to 12kg rearfacing, which usualy takes them until at least 12 months.
That said, I turned 'early', about 7-8 months. Would have liked to have left her rearfacing for longer but Jazz was a boofer, and her seat only allowed to 8kg rearfacing and at the time I didn't realise that this was quite a low weight and there were others that allowed a higher weight.
In Sweden it's recommended to 4 years, but they also have specially designed car seats that go up to 25kg rear facing.
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