I was wondering about this myself, and I found a study on the internet conducted by the RTA and the NRMA. The study found that they can go into a booster at 14kgs, but the absolute safest option is a carseat. They said kids should stay in the carseat until they are literally too big for it, ie. their shoulders are too wide. If you do get a booster seat, it must have a back, and the highest, widest sides you can find, for side impact protection. The study also said that the booster only protects them in an accident if they are sitting properly in it, ie. haven't falen asleep and slumped down. This is another reason to make sure they have a proper harness, to hold them in place as much as possible.
Hope this info helps- I'll certainly be keeping my toddler in a proper carseat as long as I can!
ps. the study also said that boosters, provided the child is big/heavy enough, are fine as second car or occasional carer such as grandparents options.
Ellie, NSW, DD 3 yrs & TTC # 2