New Zealand Prime Minister Jacinda Ardern's government is facing its first legal challenge over recent decisions including its move to no longer offer new offshore exploration blocks.

The legal challenge is by Auckland-based oil and gas producer Greymouth Petroleum, which said it filed claims in the High Court in April this year following the government's controversial and unexpected decision to stop issuing new offshore permits.

One of Greymouth's claims is that the decision, handed down 12 April, was unlawful.