As far as I understand we can use Amazon US until the start of July when the new tax laws take effect. I don't see the problem with putting in a pre-order before then.
I think it's more to the point that Amazon don't see it as worth their effort. I don't remember the numbers but their Australian business through the US site makes up a % or less of their business. and it's not just the programming of the sales tax, it's the management and reporting and returning of those dollars to the Australian Government.
Amazon probably sees development of the Australian store/site as being a better way to make money overall.