If your turnover for the next 12 months will be under $60,000 or you have ceased trading, we can cancel your GST registration.
Inland Revenue requires that you prepare a 'final return'. This final return needs to include declaring GST on the market value of the assets that are being kept in the business, or have been given away or sold. Technically this should be valued by a valuer. However, a more practical method is looking at TradeMe listings for a good approximation.
It's always best to wait until your financial statements are completed - your accountant may provide adjustments needed, and these can be included in the final return.
You can de-register from GST directly from myIR. Here is the IRD's website explaining the exact process.