It states on GN 2018/1 "You can have a maximum of $15,000 of eligible contributions from any one financial year counted towards what can be released under the FHSS scheme, up to a total of $30,000 across all years", does it means I can withdraw $15,000 before July 1 and another $15,000 after July 1?
No, only $15,000 of the amount that was transferred from your kiwisaver will count as an eligible contribution amount. It's only the first $15,000 of contributions that are paid into a super account in a financial year that can be released under FHSS. Any amounts contributed above $15,000 can not be released under FHSS.
So if you haven't made any other personal contributions in other financial years, the available amount under FHSS will be $15,000.
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The money you transferred counts as a contribution for that year which can only be used as a maximum of $15,000. If you contribute another $15,000 in the next financial year you will be able to apply to release a maximum of $30,000.