Franking credits are (as of 2000) "refundable" in the sense described in the tax credit article. This means that not only can they reduce a taxpayer's total tax liability to zero, but any excess is refunded. For example an individual with income below the tax-free threshold ($6000 in 2006) pays no tax at all and can get franking credits back in full as cash, at the end of the year.
Prior to 1 July 2000 franking credits were "wasting", any excess over one's total tax payable was lost. For example an individual at that time paying no tax could get nothing back, they merely kept the cash part of the dividend received.