I did not use that tax offset long time ago. It cost money. I just setup a floating loan i.e. nz$20,000 and if I have extra cash, I just pay back to that floating loan.
I think that .409 is great. Less is better than more.
Don't worry about refinance, you have to do it once contract expired. I did these contracts for 8 years. It was fun. The bank officer gave me lots of discount each time to let me stay with the bank. i.e. reduced official rate, extra reward cash etc.