I know that in the safe and sound seat we used to have, the 5 point harness was only safe up until 18kgs. Has that changed? Are they only going by height now? That seems crazy to me. As soon as I found out about the 18kg limit, I took ds (I think he was around 21kgs at the time) straight out of it. He would have only been 3 or 4 years old though?
I really don't know anymore Ginger - this is from the ACRI site the FAQ section- the quotes contradict themselves in my opinion - see bold sections -

Q: What does a harness mean?
A Harness is a safety restraint constructed from belt webbing / straps. There are a number of styles.

Integral Harness.

A '5 or 6 point Safety Harness' is constructed into an infant / child restraint offering two upper body / shoulder straps, crotch and hip level straps, usually connecting at the front using a buckle.

Additional Harness.

One type often called 'Child Safety Harness', also known as an 'H' Harness is an additional device that is used with a seat belt at the lap area. It needs to be connected to a child restraint anchorage as well. NB: Be careful in using this product. See adjustment Q below!

A 'Protecta' harness is a more recent development of this theme which works similarly to a 'H' harness except that the adjustment is in front of the child, secured under a cover. ( Benefit: More likely to fit where the anchor is very close to the car seat back. i.e.; 'short hook up' situations)

Additionally, there may be a circumstance where using it through the slots in the later model Safe n Sound 'Maxi Rider' or Hi Liner Boosters and other similar products may offer an advantage. NB: Be careful in using this product, you need to take time in getting any harness adjusted correctly. See adjustment Q below!


Q: What is the correct adjustment of my additional harness? (ie: Not applicable for integral harness, only for 'H' or Protecta Harness applications)
Firstly, adjust the lap seat belt section as tight and comfortable as possible, and secondly, remove all slack or looseness from the upper tether strap adjuster second. NB: Do not stretch the seat belt up away from the hips or thighs with this adjustment.
No slack, looseness or twists in any of the straps or belts.

Q: When should my children use a harness?
Children should always have upper body restraint. Discussions about harness use can be confusing. So first check the previous two answers above:

While a child can remain using a restraint system which has an integral harness they should remain in it. (NB: Currently all integral harness systems are limited to 18Kgs. Do not be confused by multi use child restraint/booster products. The integral harnesses stated safe use limit is 18Kgs.

The belief that additional harnesses should be used after this point once served our community better, when our vehicles were simpler and lap only seat belts were more prevalent. Many of today's modern vehicles don't allow the easy, safe use of additional harness systems.
The belief that 'a product', in this case an additional harness, is safer than any other product or practice is often misleading. Using these products safely is reliant on the correct adjustment and daily use. To instal and then expect this product to provide an on going safe environment is unrealistic. Users of Harness systems must be able to understand, install and / or adjust them appropriately as required, and only then can they be considered a safe option.
Then later on it says

b. Child Restraints

Child restraints are forward facing 'child seats' or 'car seats' which are secured to the vehicle and also come in two types. Both types incorporate a '6 point' harness system that secures the child.

1.
Dedicated 'Single purpose child restraints' these have been come less available over the years as the convertible choices have expanded: (These have been available for toddler use up to 18Kgs for many years, but any 'Post 2010' Australian Standard compliant product is lmited by height marker only.)

Products in this category are and or have included the Toddler Rider, Cosi, Series 3, Maxi Cosi 'Complete Air' and Discovery plus.

2.
Combination child restraints: (ie: A toddler car seat with integral (built in) harness which can be converted at a later stage to be used as a booster seat with which the older child can use the vehicles seat belt. (NB: Post 2010 Aust Standard product is height marker only limited)

Eg's: Maxi Rider, Exeed, Swish air, Explorer. to name a few. (NB: Vehicle compatibility clarification is necessary to determine suitability of employing an additional harness system for booster stage use).

Combination types have been available for toddler use up to 18Kgs and older child 26Kgs for many years, but any 'Post 2010' Australian Standard compliant product is lmited by height marker only). NB: This type of booster must be anchored at all times.
So in one part it says integral harness only rated to 18kg, then it says later on is limited by height marker only.

I have no idea - I currently have an 24kg DD in a convertible but not in booster mode (so wearing the integral harness) and can find no clear guidelines whether she is safer in this mode or booster mode. I also have an 18.2kg DS in an Compaq Deluxe which he is clearly too big for. So I am definitely one of the 80% with incorrectly restrained children. DS - I was aware I needed to change and that is what started all these investigations.

I mean I can ask companies on FB and post in FB groups but really it shouldn't have to be like that - it should be clear for a child of a particular, age, weight, and height what are the most appropriate restraints. Why is there not a table somewhere which lists the change to booster heights on all the different seats (if indeed I can go by height alone) - why not make it easier for people (is no wonder children are moved to boosters earlier than necessary because the situation is so confusing - manufacturers recommend you follow the switch guidelines (but you can find info saying they have been checked to the highest harness marker, as well as info saying harnesses have been checked up to 32kg).