How the repayment is worked out
The monthly repayment is the loan multiplied by the monthly interest rate, divided by one minus one plus that rate raised to the power of minus the number of months. Weekly and fortnightly figures are the same yearly total split into 52 or 26 payments, so a fortnightly payment is easier to budget for than a monthly one. The yearly total is the same either way. This calculator doesn't model extra payments, or the small saving that paying more often can make over the term.
Real loans have fees, and rates change. Most fixed-rate terms run for one to five years, after which the rate resets to whatever the lender offers then. Use the calculator to compare loan sizes and terms, and then check the figures with your lender before you rely on them.
A worked example
Borrowing $600,000 at 6% over 30 years costs about $3,597 a month, or about $830 a week on the same yearly total. Over the full 30 years you'd pay roughly $695,000 in interest, more than the amount you borrowed. Over 25 years the monthly repayment rises to about $3,866, but the total interest falls to roughly $560,000. A shorter term costs more each month and far less overall, so the right term depends on what you can comfortably afford.
Questions to ask your lender
Ask whether the rate is fixed or floating, and what a break fee would be if you repay or switch part way through a fixed term. Ask how much you can borrow once they've looked at your income, your debts and your living costs, because that figure is often lower than the one you had in mind. And ask whether you can make extra payments without penalty, since that is where most of the interest saving comes from.