There are offset accounts which have no fees, I have one with Westpac. The benefits of an offset are to reduce the interest your paying on your mtg. You would actually earn % your paying on your mtg at say 7% which you'd never be able to earn in a savings account and then you'd be taxed on your earnings.
The one I'm looking to go with is Bankwest's Super Start Home Loan, only 6.9% interest and with other great deals. But because this package provides some benefits, it cost some maintenance fee for its offset account.