I have just updated the link in this article that I wrote in August last year. The link takes you to an Australian website with crash test data on child restraints including booster seats. Hope it helps
cheers
Kim

You can buy a harness for booster seats and normal car seats from Big W, Kmart and Target. Check the safety area. They're hard to come by so get them to order one in for you

Hi there

Jenaya has been in a booster seat for about 6mth now as her head was above the top of her car seat. I feel that car seats are safer and have absolutely no reason why, only that I do. I didn't think she was safe enough just in a booster with the car seat belt so I bought a harness that the car seat belt goes through before buckling up. Really tho, with me I don't rekon any type of harness/belt will make me think she is safe enough!! lol

My 4.5 yr old still fits in his car seat (safe and sound converttable) and will be moving to one of the Maxi rider ones in a few mths. With the weight range of them he will probably still be inone unti hes around 9 (we have a very short/small family)
IMO boosters just aren't enough unless they have a harness but they are still better then just having the kid in an adult seatbelt.

I have not seen any booster with a 5-point, probably because it's just that...a booster.  It's really made to lift your child up so the regualr seat belt crosses their body in the right position.  I would not go on the weight alone for this.  If your child is tall enough and then if you feel comfortable switch them.  I would keep in the car seat as long as possible.  In the United States our children have to be over 40 lbs and a certain height to be in a booster and over 60lbs and a certain height to be just in a seat belt.  And we can get pulled over by the police if they are not propperly restrained.  For me I will keep my baby in a car seat as long as possible as it does provide the best protection with whip-lash and other seat belt related injuries. 

Koen still fits in his mothers choice brand reversable car seat (facing forwards lol)
but coz he is growing fast, i have a booster seat ready for him, im sure you can buy 5 point harnesses and they fit any booster seat as mine was $70 for the booster and theres instructions on fitting a harness underneath (sold separately) im yet to get one as they go like hot cakes! im sure you can use the same bolt that is already in your parcel shelf..

xx chan

have a look at this page it is from baby co and of car restraints for 8kg to 26kgs and they have the harness

http://www.babyco.com.au/catalog/product.asp?pID=197&cID=32

hope that this helps

I have a Maxi Rider convertible booster. It has a 6 point harness (the strap between the legs is split in 2 parts for comfort and safety) and that harness goes up to 18kgs.  The next harness up is a little difficult to explain but can actually be used as a 5-point harness.  I had to go in to the baby shop to have them show it to me, so you might need to do that as I can't think of how to explain in words hehehe.  There are other types of convertible boosters from 8kgs to 26kgs.  My daughter is nearly 4 and won't move from her Maxi Rider until she is much older.  She's still in the original harness and is 14kgs so has a long way to go before outgrowing it.