Technically, we do not have capital gains tax in New Zealand.
We do however have a system that taxes gains that would otherwise have been on capital account (and non-taxable) if there had not been an intention to make that gain at the time that the capital asset was acquired. Thus for example, if at the time of purchasing a property, the principle intention for the purchase was to sell that property in the future to make a profit, any such profit will be taxable.
The same situation can exist with buying and selling shares in public companies.
The reason for this is that our taxation legislation seeks to tax all profit where there existed an intention to make such profits. In the case of capital assets which are purchased with the intention of sale, our taxation laws treats such profits as ordinary business profits and not capital (or tax-free) gains.
The Income Tax Act is quite explicit in taxing profits made from certain sales of land. Over and above the comments made above about profit-making intentions, the Act taxes all land associated profits made by builders, and those involved in sub-divisions (unless the land is owned for more than 10 years and work undertaken to give effect to the subdivision is of only a minor nature). The law in this area can be very complex (including a vast amount of case law) and any given situation where a profit might have been generated needs to be considered on its own merits to determine whether or not that profit is likely to be subject to income tax.
IRD has been aware for some time that there has been much activity over recent years on land speculation resulting in profits which have not been returned by taxpayers as taxable income. IRD has a special group set up, ostensibly to educate taxpayers and those involved in the real estate industry, which is reviewing land transactions of the recent past, with the intent of capturing into the tax net, the profits that have up to now, intentionally or otherwise escaped capture.