You’re required to register for GST if:

turnover from your property transactions (and other transactions) is more than the $75,000 GST registration threshold, and
your activities are regarded as an enterprise.

Your activities may be regarded as an enterprise if you:

sell new residential premises or potential residential land
buy land with the intention of developing it for immediate resale at a profit
develop a property to sell.

A one off transaction can still be considered to be an enterprise.

Please call to remove any confusions regarding your property transaction and its Capital Gains and GST implications, if any.