Your income, and how much of it counts
Base salary or wages are the easiest part. If your pay is steady and you have been in the job a while, lenders will usually count it in full.
Overtime, bonuses, commission and second jobs are treated more carefully. Because that income can stop, lenders generally want to see a history of it before they count it, and they often count only part of it.
If you are self-employed or contracting, lenders usually ask for one to two years of accounts or tax returns so they can see a consistent pattern rather than one good year.
Debts and repayments you already have
Every regular repayment reduces the room left for a mortgage: personal loans, car finance, buy-now-pay-later arrangements and hire purchase all count.
Student loan repayments are taken straight from your pay, so lenders treat them as a fixed reduction in your take-home income rather than something you can pause.
Credit card limits, not balances
This one surprises people. Lenders look at the limit on your cards and overdrafts, not the amount you currently owe, because you could draw the full limit at any time.
So a card you clear every month can still reduce your borrowing power. If you do not use the limit, reducing or closing it before you apply can help.
Living costs and dependants
Lenders look at what you actually spend — groceries, transport, insurance, power, childcare, subscriptions — rather than assuming you will suddenly live on less once you have a mortgage.
Children and other dependants add to those costs, which is why two households on identical incomes can end up with very different numbers.
Your deposit or equity
Your deposit sets how much of the property's value you need to borrow. The bigger the deposit, the smaller the loan and the lower the risk from the lender's point of view.
If you already own a home, the equity you hold plays the same role. Either way, where the money came from matters — lenders want to see genuine savings, a KiwiSaver first-home withdrawal or gifted funds clearly documented.
Where to from here
You do not need perfect numbers to get a useful picture. Run your own details through the calculator and you will see a borrowing range plus the things helping and hurting it.