Just my 2c. I did some property investment seminars a few years back and one thing I remember hearing was that as a rule you should fix below 7.6% for as long as you can.
When ours came up the interest rates were really low and I got 6.75 i think for five years. As life turned out we ended up selling that house long before the 5 year period was up and I am now renting. Because the interest rate was lower than the interest rates at the time of sale, the penalties for exiting early were minimal.
I like what people are saying about revolving credits saving you money if you are disciplined. If you know yourself to not be so disciplined with money, perhaps the BNZ scheme where your savings accounts reduce your mortgage interest is better for you - especially if they can give you a savings account where you get penalized for unnecessary withdrawals.