Short answer: Yes if it covers the full deposit. Most people combine it with savings.
Only if your KiwiSaver balance covers the full deposit amount. For a 5% deposit on a $500,000 house, you need $25,000. If your KiwiSaver has exactly that, yes. But most people combine KiwiSaver with savings. The bank just wants the total deposit. They do not care which account it comes from.
House Price: $500,000
Five Percent Deposit: $25,000
Kiwi Saver Balance: $30,000
Result: Yes, KiwiSaver covers full 5% deposit