Every progression from (1) Rearward facing to (2) Forward facing to (3) a booster to (4) an adult seatbelt, is a step backward in safety. Sounds drastic, but keep in mind they start off being much safer than you, and end up being as safe as you.
Knowing that rearward facing is safest for babies, it makes sense to stay that way for as long as your seat allows – check the height markers sewn on the fabric of the seat. When buying a seat, make sure it can stay rearward facing for at least 1 year, preferably 2 or even 3 years.
Once your child is forward facing, they must stay in this type of harnessed seat until they turn 4 at a minimum. If your child fits their seat, keep them in it. Smaller children may be 5 or even 6 before they need to progress to a booster.
The RMS, the NRMA (and Pearce’s) recommend you boost your child until they are 145cms tall. This is effectively most of their primary school years. They can be legally out of a booster at 7, but on average they are 20+cms too short to safely use an adult seatbelt. I will explain why next edition.