For income tax purpose, cryptocurrency is treated as property, not currency.
It depends on your purpose for acquiring the cryptocurrencies, when you acquired them for the purpose of disposal (resell or exchange), the profit from selling it is taxable income.
Because bitcoin and similar cryptocurrencies don’t produce any income stream or benefits, IRD take a view that cryptocurrencies are generally acquired for the purpose to sell. When you swap one type of cryptocurrency for another, or exchange cryptocurrency for New Zealand dollars or other currencies (such as US dollars or Euros), you will have to record the realized gains as taxable income at the time when it occurs.
Conversion rates must be from a reputable exchange with a reasonable trading volume, and standard seven-year recordkeeping requirements apply. You should be able to access to your transaction history (deposits, withdraws and transactions) and be able to export this in a csv file; bank statements and cryptocurrency wallet addresses are also required to be retained.
If you are a cryptocurrency miner, you should be aware that cryptocurrency mining is not a hobby! It is classified as an activity aims at making profits – all mining-related fees or rewards are taxable income.