The number that matters isn't the repayment
A $600,000 loan at 5.75% over 30 years costs about $3,501 a month — manageable. What the marketing never leads with: you'll repay roughly $660,000 in interest on top of the principal. More than the house cost. That's why the levers below, which look small monthly, are worth six figures over a loan's life.
The three levers
Rate. Every 0.25% off a $600,000 loan saves about $90 a month and $33,000 over the term. Refinancing when your rate drifts 0.5%+ above the market is the single highest-value hour of admin in personal finance.
Term. Stretching from 25 to 30 years cuts the repayment by ~9% but adds ~20% more total interest. Take the 30-year term for flexibility if you must — then pay it like a 25.
Frequency. True fortnightly repayments (half the monthly amount, 26 times a year) sneak in one extra monthly payment per year. On the loan above, that alone clears the debt about 4 years early. Some lenders quote "fortnightly" as monthly×12÷26, which does nothing — check which yours uses.
Interest-only: cheaper now, dearer always
Interest-only repayments on the same loan are about $2,875/month — $626 less. But the principal doesn't move, rates on IO loans are typically 0.2–0.6% higher, and when the IO period ends you must repay the full balance over the remaining term, so repayments jump sharply. It's a cash-flow tool for investors (whose interest is deductible), rarely a good idea for owner-occupiers.