An actual surplus not a budget surplus. A budget surplus has nothing to do with an actual monetary surplus. (Not that you'd know it with the amount of LNP sheep that still think we had no debt under howard and love to try to brag about it on the internet.)
We already have savings in the form of the commonwealth futures fund, (set up by howard administration) whos book is currently valued at ~$100bn so any savings go in there.
The downside is it's only purpose was to pay the superannuation of public servants... i.e. themselves lel.
If this happens during times of debt, imagine what would happen in a surplus.... we'd probably just see it all pissed away on MP and shadow MP salaries / new houses and artwork for their houses/offices.
Incidentally, the fund was modified by the Swan (using treasurer's name as I cant remember which of the 700 Labor PM's it was under) to include funds for hospitals, education, welfare etc.