Short answer: At least 3 years of contributions. No fixed dollar amount, but more is better.
You need at least 3 years of contributions to withdraw your KiwiSaver for a first home. Most lenders want to see you have been contributing consistently. There is no fixed dollar amount, but the more you have, the smaller your mortgage will be.
Monthly Contribution: $200
Employer Match: $150
Government Contribution: $521/year
After Three Years: ~$13,563