Hi BobbyBob (We've chatted on another thread). I'm glad to see other parents taking car seat issues so seriously.
Farmers, Baby Factory and Baby City in NZ all have Safe n Sound car seats for children up to 25kg (Farmers has some really good prices, probably based on superior buying power). These use tether & bolt just like the ones for smaller babies, and also have the 5 point harness. IN fact the one we have can go from infant (rearward facing) up to 25kg (becomes forward facing at some stage, can't remember exactly when. It has sitting and reclining positions (great for toddlers to fall asleep in).
There is also a brand that looks more like the foam rubber booster seat (but with the 5-point harness) in that it does not have the "wing" type sides of "baby" car seats. I considered buying this for my DS, but am so glad I did not. We were involved in a collision where another driver fell asleep at the wheel and hit our car side-on. The "wing" sides protected DS, who I am convinced might have had a neck or back injury without this feature. DS was in a Safe n Sound car-seat as described above, and was unharmed by the accident (thank God). Although he is heavy enough by the regulations to travel in one of those-foam rubber type seats, if he had been in one of those he would proabably have been injured.
Our second baby is due in Sept, and we'll be getting another Safe n Sound car seat for him/her.
Baby Factory & Baby City also have other makes doing a similar job. The flimsy foam rubber types are also available from these stores (and places like the Warehouse etc), and I agree make me very concerned about safety.
If you go in to the shops mentioned above, the staff are usually able to advise what weight is the maximum for the different types of car-seats. They can also advise on various features (saves you from trying to come to grips with manuals of 20 different types of car seats on display). When I was hunting for a car seat I took a pen and notebook, because most seats have different specifications, and I found it good to be able to compare the safety features that I considered not-negotiable. It was a bit time consuming, but very worthwhile.
Some of the Safe n Sounds have a great feature of a push-button retracting mechanism for tightening the staps (good for when you've just taken off a bulky jacket and need to quickly adjust the straps to the right length). We chose this feature mainly because the grandparents could not figure out how to adjust the straps on some of the other types of seats.
hope this helps. Let us know how you go.