Hi, the NZ tax year finishes on 31st March. See the tax years below.
NZ tax years:
2019 Tax Year = April 2018 to March 2019
2018 Tax Year = April 2017 to March 2018
2019 Tax Year = April 2016 to March 2017
If you wish to check up about your previous tax years (eg. 2017 tax year or 2018) The best thing is probably to make an online account with the IRD and you can send them emails through there. You can also check your tax balances for previous years. https://myir.ird.govt.nz/eservices/home/_/ You can also call the IRD on 0800 227 771 and tell them what you want.