Thanks for your replies.
The seat I bought is a single and has an anchor point in it which is good cause I don't think I'll put DS in a carseat/booster but will use a H harness.
We will only need to use it if DH comes in the car with us, otherwise DD1 will sit in the front and the 3 little kids can sit in the 2nd row - unless DS really wants to sit in the third row!
I rang a few places today and no one fits them.... One guy did but does it as a 'backyard' type thing, he charges $300 but doesn't supply a certificate or anything. If I get it don't 'properly' by an engineer ive heard it will cost around $1000!!! I know it's still cheaper than buying a new car but sheesh.
I've emailed Vic roads and just waiting for them to get back to me with the legal jargon and other fitting stations for Vic.