A challenge to the Government's 2018 announcement that it would immediately stop offering offshore oil and gas exploration permits begins today, with a gas producer claiming the move breached the rule of law.

Greymouth Gas Turangi Limited is suing the Minister of Energy and Resources Megan Woods over the way the Government gave effect to the offshore permit ban, as well as the refusal of the company's application for a permit in Taranaki in 2017.

The case is being heard in the High Court in Wellington.

Greymouth Gas boss Mark Dunphy told Mike Hosking the ban shouldn't have come into effect until the law was changed.

"The rule of law is an important thing. One would think that the most important people who need to comply with the law will be the Prime Minister and her ministers.

"What goes around, comes around."

He says they've been seeking a permit in Taranaki, and the government is obliged to grant it.

"It's likely to be about two to three years worth of gas for the nation, a couple hundred million dollars in value, so it's no trifle."