Solo Convertible Kids Booster Seat (6mo to 8yr) $109 Delivered @ Amazon AU
This may be the cheapest price for a brand new booster seat that covers a wide age group + delivery is free.
Don't forget cashback via cashrewards.com.au or shopback.com.au.
Item description from the page:
- Large side wings for maximum side impact protection
- 5 Adjustable headrest positions
- Reclining backrest to fit a wider variety of vehicles
- Seat sash belt guide to keep belt away from child's neck
- Great fabric design with easy to remove machine washable covers
Remember to get it fixed in the car by an authorised mechanic (list can be found on the respective state's RMS website) - safety is very important.