The treaty only covers nuclear weapon capable heavy bombers. So, in principle, there would be no breach of treaty terms should America export an non-nuclear version of the B-21. Basicly lacking the weapons integration for said weapons. Maybe change the brand of coffee percolated installed too.
But the subject is moot. Australia simply can’t afford it. Here is a good piece on the need to develop a strategic economy doctrine as part of a comprehensive national security plan.
Australia's Economic Security: Is there a problem? - Australian Defence Magazine
The Australian economy has been travelling forward on inertia since Gillard took over. Issues like natural gas prices, electricity prices, rising household debt levels, leases of ports, sliding educational outcomes, etc are indicative of policy without direction. Something the article doesn’t mention is increasing scarcity of water supply relative to demand. We have reached the point of diminishing returns from immigration as a driver of growth, since from now on, increasing water costs, thus business and living costs, will cancel out increased growth.
The article then discusses global economic risks, questions local economy grow predictions and raises questions about the sustainability of defence spending.