You cant pre-pay the interest on a variable rate loan, or you can, but you cant claim it as a deduction like you would a fixed rate.
Interest in advance fixed rate loans are a little bit cheaper than normal fixed rates, this is either a happy coincidence, or because if it were the same interest rate, the ATO would argue the only reason you were pre-paying was to lessen your tax, which your not supposed to do.
Just dumping a years interest in a variable rate loan is not pre-paying interest, and is only tax deductable for the year it occurred (not for the following tax year or part thereof).
I spoke to NAB yesterday and was told it was too late to change it from variable right now, but when I go in to sign the papers late this week, they'll set it up to switch to a fixed loan for 12 months so I can pre-pay before the end of June. All good. Thanks for the advice everyone.