When a property has been bought with the firm intention of resale you'll have to pay tax on any profit from the sale. The intention to sell does not need to be the main reason for buying the property, it could be one of a number of reasons for buying.
Moana buys a property with the intention of providing a home for herself and her children
When Moana eventually sells her home she hopes to make a gain and leave her children a legacy.
In a year's time she gets a new job and decides to sell the property to move closer to her new job. Property prices have risen, so Moana is able to sell the house for much more than she paid for it and can buy a bigger home.
The gain from the sale of Moana's property is not taxable as her intention was always to provide a home for her family. If Moana bought and sold the property within an applicable bright-line period, she needs to consider the bright-line rule including any exclusion that may apply. Because the property was used as her main home the whole time during the bright-line period, she is eligible for the bright-line main home exclusion. This means any profit from the sale is not taxable.
Sue buys a property with the intention of selling it for a higher price when the time is right
Sue and her family decide they like the area the property is in so they live in it in the meantime. Sue has a regular pattern of buying and selling residential properties while living in them.
Two years later, house prices in Sue's area have risen to a level where she could make a good profit on the sale of her property and she decides to sell.
The sale of Sue's property will be taxable, because her intention at the time she purchased it was to sell it, and because she has a regular pattern of buying and selling residential property. Sue declares the profit on the sale as income on her IR3 tax return.
Frances and Bruce buy a second property in the hope that it will quickly gain in value. They decide to rent it out in the meantime. One of their reasons for buying the property is to sell it and make a gain from any increase in its value.
When they do sell - Frances and Bruce must account for the profit in their annual IR3 tax returns, because part of their intention when they bought the property was to sell it.