You cannot use your KiwiSaver account to start a business. There are only a few instances in which you can withdraw your KiwiSaver savings including for well known reasons such as a First Home KiwiSaver Withdrawal and a Retirement Withdrawal (age 65), as well as lesser known reasons such as experiencing Significant Financial Hardship, permanently emigrating to a country other than Australia, being diagnosed with a serious or terminal illness, and having a Life-Shortening Congenital Condition.
