Would it matter if you split delivery between two different addresses? Make two separate orders at the same time, but have one delivered home, one delivered to project site address?Basically, keep orders below AUD$1000.
Big orders, break them down into smaller orders spaced a few days or a week apart.
Some parts for vehicles over 30 years old don't get customs duty.
Duty is different for petrol vs diesel engine parts. If an order is above $1000, say it's for a 30+ year old petrol vehicle.
Basically, stay under $1000