Currently the major banks are offering fixed rates that are between 25% and 75% less than the floating rate, e.g. at the BNZ the standard floating rate is 5.99% and the one year fixed rate is 5.25%. At the ASB, the variable/floating rate is 5.75%, and the one year fixed rate is 5.45%. ASB also have a second mortgage rate of 8.75% (although there isn’t much information about this product and its criteria on their website). Kiwibank has a one year fixed rate of 5.25% (4.99% if you have at least 30% equity).
(Note: Interest rates are as at the time of writing).