Australian tax is precalculated.
In Australia, tax is not added on to the bill. It is already included in the advertised/displayed price.
For example: In America you could pay $20 for something and get a tax added on your invoice making the total charge higher like $24.
In Australia this is not the case. If you see $20 for something, that already includes the tax and you will get an invoice for $20. The tax is still noted on the invoice though so the billing system should calculate tax differently on the invoice. I think this is also true for New Zealand.
You should take the $20 (f), calculate the tax based on the percentage (20 * .1 = t), subtract it from the total (f - t = s)
Then show: Total before GST (s), and total after GST (f). Australian invoices are also required to show the registered ABN (Australian Business Number) on the invoice.
GST and an ABN (Required to submit taxes.) are only required for businesses if their current GST turnover (their turnover for the current month and the previous 11 months) totals $75,000 or more ($150,000 or more for non-profit organisations) or if their projected GST turnover (their total turnover for the current month and the next 11 months) is likely to be $75,000 or more than ($150,000 or more for non-profit organisations).
An ABN is also required if a business is a tax concession charity or a deductable gift recipient but that probably won't be a problem here.)
This means tax is optional for startups until they earn a certain amount so there should be an option to turn it off.
This also means your billing system can't legally be used in Australia (and probably New Zealand) unless it is customised to handle this system. Please change invoice generation for these countries to take this in to account.
Instead of handling things differently per country, you could probably just add a check box for pre-calculated in "Add Tax Type" and handle things the Australian way if the boolean is on for that tax type.
The BSB display issue could be resolved if we were just able to customise the bill output and add custom data, say below the logo for example or change the bill generator so we can customise using a billing template.