Banks offer better mortgage rates (and cash backs) to customers that have at least a 20% deposit. You must have at least a 10% deposit to access your KiwiSaver, so this is where most lenders draw the line. Banks are permitted to have 15% of their owner-occupied borrowers with less than 20% deposit. The cut-off for rental properties is a 30% deposit.
New properties are exempt from Reserve Bank restrictions, making it possible to buy a new property with as little as a 5% deposit, but the issue with KiwiSaver still applies.