Yes, labor market competition is the primary factor. But is there a place in Australia where engineers can work for higher salaries and the same or a lower cost of living?
In the US, the discrepancy between Bay Area and non-Bay Area salaries is not as great as they seem at first glance, once the real cost of living is factored in.
That's the same situation as in Sydney, but Aussie engineers aren't getting near the level of pay of Bay Area engineers.
It's not the housing prices, it's more the labour market competition.