At 4 my DD1 is still only 16kg, so a harness til 25 kg will do her until she is at least 6. A 18kg limit would put her about 5.5 before she would go to a seat belt. She is super tall and went into a booster by 3 tjough as she was too tall for all the Australian seats.
It would be illegal now to have a 3year old in a booster with adult belt as they are required to stay harnessed until 4. Australia needs to wake up and provide appropriate seats!
I think you mentioned LATCH being brought in in an earlier post. ISOFIX is being written into the standards now. It should be ready to go 2013. But this doesn't mean European(ISOFIX) or US (LATCH) seats will be legal here then. All it means is Australian manufacturers can produce them and sell them to install with ISOFIX, it doesn't mean we'll get extended rear-facing seats. There is campaigning to have type D seats (to rear-face up to 4years) manufactured for sale in Australia and also to have international ERF seats approved for use here. But it is very slow going.
Australia is very stubborn when it comes to car seats it drives me nuts. It's a lot to with protecting the Australian market my opinion of most Australian car seat manufacturers is pretty low after having contact with them about this. With the exception of Safe n Sound, they are making all the right noises so far about making a decent seat but are restricted by design and testing requirements here

Oh and the FF harnessed weight limit on the European seats was more for Ginger, she's not looking to ERF she wants to extended harness. carseat dot se is great for ERF not so good for extended harnessing. The Radians are good for both