It depends on how you view the affordability of the payments now and how much of your working capital you have tied up in the investment property. If you cannot live with increased mortgage payments, then fixing offers protection, but fixed rates are generally the most inflexible, and once you know the swap rate for the bank, generally the most expensive also.
On an interest only basis also you are depending on a rising market to clear the capital repayment on the house which is probably Ok long term (>7 years on average) and makes sense from a tax point of view. On the other hand coming near a general election at the top of a property cycle where first time buyers are priced out of the market, perhaps you should bear in mind the housing strategy of the next government could change the rules on taxation on rental income versus interest paid on a mortgage for investors, and gradual interest rate increases and capital gains tax changes could stagnate the resedential property market for some time to come. On that basis make sure you have invested wisely the capital portion of the mortgage which you are not paying off now, and make sure you try to get a better return on that versus what you will potentially loose on the cost of a fixed term mortgage, if you really need to fix.