Tax File Numbers are like driving license numbers. You get one, and it's yours for life.
Most Aussie expats have lived and worked in Australia at some stage, thus, they have a TFN. You could open a bank account and stuff all your money in that account, and not tell the bank your TFN, but all the bank does then is tax the interest at the highest marginal rate, which is something like $0.48 in the dollar, on every account where a TFN hasn't been supplied.
So, how would you suggest an Australian lives, works, banks, and retires, without a TFN?