The fact you are on a very good tracker might make it less simple than it otherwise would be - banks lose money on trackers so the quicker they are paid off, the better for the bank (and vice versa in what you are hoping to do). Are you young enough that you still have plenty of working years to extend past 13 years (18 years seems quite a short repayment period to have selected in the first place - why was that)?
That said, if you can't pay, you can't pay - so an extended term is better for the bank than interest only or less than interest only. After the budget, you should set out your household budget (maybe get help from MABS) and present it to the bank with your preferred course of action. They might resist but as I said above, a slower repayment of capital is better than no repayment of capital or you slipping into interest arrears.