Barnaby Joyce must be kicking himself for missing this ridiculously simple way to confirm he was a New Zealander.

The NZ government's website features a quick test entitled: "Check if you are a citizen." It also suggests you ask the Citizenship Office if you are still not sure.

The online test asks whether you were born in New Zealand or Samoa, or whether one of your parents was an NZ citizen when you were born. The latter is the case for the Deputy Prime Minister, whose father James was born in Dunedin on the south island.

This means Mr Joyce is a New Zealand citizen by descent, the short test confirms.

Barnaby Joyce could have verified he is a citizen using this simple test.
Barnaby Joyce could have verified he is a citizen using this simple test.

You can register your citizenship and order a passport, the website suggests - but even if you don't, you are still a citizen.

Mr Joyce must have had a terrible sinking feeling when the link was emailed to his media adviser by Fairfax Media a week ago.

Today, he revealed in Parliament he may be a dual citizen and would refer himself to the High Court, and authorities in New Zealand have since confirmed the Nationals leader has dual citizenship.

Under NZ law, Mr Joyce's father was born a British subject in 1924, but became a New Zealand citizen when citizenship laws were introduced in the country in 1949. Mr Joyce was born in 1967.

The shocking revelation puts Malcolm Turnbull's majority government under threat. Not what your countrymen would call choice, Mr Joyce.