I can't think of a bank that does allow interest only repayments on anything other than monthly.
Technically it would be cheaper to go fortnightly as interest is paid in arrears and calculated daily. The difference would be tiny, but there would be a difference.
If you have an offset account against the loan, there would be no difference at all. The balance in the offset account would act against the accrued interest.
Fortnightly repayments are of greater benefit when making Principal & Interest repayments. To calculate fortnightly repayments in this scenario, the bank calculates the monthly repayment and divides it by 2. Technically there are 2.167 fortnights per month, so on fortnightly repayments you make an extra 2 fortnights repayments every year. This is where the fortnightly repayments gets you ahead, but it only works with P&I repayments.