tax right off

I'm not an accountant, but I'll venture a guess that it has nothing to do with how much land you have, and everything to do with whether you approach the deal as a profit-making farm. The days of having a "hobby farm" as a tax write-off are gone, if that's what you were thinking.
You are thinking of the non-commercial loss rules. Unless you can pass certain tests, you can not offset these losses against your other income. They simply carry forward until you make a profit from that same activity. Think of it a bit like capital losses, which you can only offset against capital gains. If your farm loss doesn't pass any of the tests, the loss will sit there until your farm makes a profit, in which case you can then offset it against that same income.
It is as ozperp and leikela say, you need to be making around 20k (gross not net) from your enterprise for the ATO to take it seriously, it isn't dependent on land size.