Congratulations, that's a huge amount in Kiwisaver for a 22 year old.
I think Kiwisaver money can only be used for a home you'll live in, rather than an investment property, but I may be wrong. Does that mean it's for a house you'll live in?
I wouldn't buy a house to live in unless I knew I was going to stick around for at least 10 years, due to the costs of buying/selling (real estate agents fees, LIM report, builders inspection, moving costs etc.).
Also, is it NZ$300K or US$300K?