Hi guys
I just found this http://www.youtube.com/watch?v=y2LFo8vVi04 which higlights the need to restrain older children (under 5) in 5 point harnesses and not booster seats with adult seatbelts.

Bit confused our booster seat has a 5 point harness which the adult seatbelt goes through?

Our daughter is 3 and can no longer fit in anything other than a booster seat.

Now I am worried

I'm confused too!
From reading around, it looks like a good booster seat should have some sort of crotch restraint, which would prevent submarining, which is where a child can slip out of the booster seat and out.
From the clip it mentioned the little girl's organs were squashed by the adult belt, I couldn't see any boosters in Target today which didn't rely on adult sashes and some sort of tether.
Here's the RAC advice http://www.racv.com.au/wps/wcm/conne...booster+seats/
Never use a booster seat with a lap belt only as this can cause serious injuries in a crash. It must be used together with a child harness or a lap sash seat belt.
Choose a booster seat with a back because it offers more protection in a crash than one without a back.
Check the seat belt is taut and firm, and is crossing the child's shoulder and not their neck, every time the restraint is used.