Australian labor costs are significantly lower than the US, as are labor costs in most countries. Americans are paid pretty well.

Solar installer costs are broadly comparable as Australians are better qualified and even if they weren't comparable the fraction of the cost isn't enough to explain the total difference.

There's various studies comparing the two countries, Tesla did one and found various technical approach changes and permitting reforms. It suggests labor is 7% of the cost in the US. Soft costs around acquisition, sales and marketing can be 18%.