H harnesses arent recommended, they are dangerous. Youll be hard pressed to find a 5 point harness accomodating up to 26kg/7 years old without importing (which is illegal).

I would personally recommend the Infa Secure 0-8 Luxi Crown. Google it. It has extended use of the 5 point harness and then converts with adult seatbelt.

0-8 Luxi Crown: Australia's only newborn to eight years child restraint.

VERSATILE
The 0 to 8 is the most versatile of any car seat. Suits the needs of: parents, grandparents, carers, childcare centres, taxi/hire companies, etc. by offering a single solution design.

COMFORT INSERTS
Multi use inserts for baby's comfort and enhanced safety. As baby grows, they are removed, making the 0 to 8 suitable through to eight years old*.

COMPACT FOR EASE OF FIT
The compact multi-recline action means the 0 to 8 in rearward facing mode fits most cars comfortably.

FIVE SHOULDER SLOT HEIGHTS
The 0 to 8 features five shoulder slot heights making it suitable for a vast range of different sized children, from premature babies through to most tall or large four year olds*. It can then be converted into a booster seat for use through to eight years old*.