Kidsafe say 8kg - not before as thats the Australian Safety Standards. The new seats can keep them rear facing upto 12kg now. They are doing it because its so much safer for baby. One thing I learnt from Kidsafe is a seat needs to be replaced once 10 years old so be careful of second hand jobs. Some-thing about the plastic its made from becoming brittle and no longer meeting safety standards. Just thought that was interesting.
Robin Barker has a book 'Baby Love' and it says in there that they need to be 9kg OR 6 months OR 70 cms. I changed mine around when DD had good head control and put it on a slight angle (the seat has provision for this). She was about 4.5 months, 4.8kg and around 65cms. She was so much happier which makes for a happier driver - ME!