Greenpeace Aotearoa executive director Russel Norman and volunteer Sara Howell face an anxious wait to find out whether they will be convicted after trying to disrupt oil exploration off the North Island's east coast.

At the end of an hour-long sentencing hearing in Napier District Court this morning, Judge Arthur Tompkins reserved a decision and remanded former Green MP Norman, 51, and Howell, 26, for sentencing on September 24, although he hoped to deliver a decision earlier.

The pair had pleaded guilty to a charge laid under a 2013 amendment to the Crown Minerals Act, after a boating protest in the path of the 21,000-tonne, Amazon Warrior about 50 nautical miles out to sea off the North Wairarapa coast on April 10 last year.

Co-offender Gavin Mulvay, who was not a Greenpeace employee or member, had accepted diversion, without a conviction, in the early stages of prosecution.

Prosecutor Cameron Stuart opposed the application for a discharge without conviction, highlighting the dangers involved as the Amazon Warrior - towing 8km of cables and equipment spanning over a kilometre wide - was forced into an eight-hour, 360-degree manoeuvre to avoid a mishap and reposition itself.

The turn had cost "hundreds of thousands of dollars", said Stuart who also cited Greenpeace media campaigning, including a media release two days ago, as reflecting a genuine lack of concern.

He said that having initially pleaded guilty, the pair reversed their stance after the setting of a date for trial in April, which he said recognised they had no defence.

Defence lawyer Ron Mansfield said prosecution claims of risk to safety were an overstatement, and the incident was "low-level civil disobedience" aimed to bring about change, for the benefit of the community and future generations.

The legislation had been introduced by the last National government to stop disruption, but the new Government was not issuing such exploration licences.

Stuart said a conviction would not have undue impact on the two defendants, but Howell, from Ireland, had her immigration status at risk, with a visa extension already revoked, but granted an extension,. Mansfield said the potential impacts and possible limitations on travel were reasons Mulvay accepted diversion to avoid detection.

The offence, with the prosecution being the first under a law change, carries maximum possible penalties of one year's imprisonment or a fine of $50,000.

Mansfield said the pair were prepared to donate $3000 each to Coastguard New Zealand as part of the outcome if there was no conviction.

Russel Norman in front of the 125m Amazon Warrior nicknamed 'The Beast', conducting offshore oil exploration vessel off the New Zealand coast on behalf of Statoil and Chevron. (Photo / Supplied)

Today's hearing was accompanied with more banner and placard bearing supporters outside.

Greenpeace used the crowdfunded boat Taitu in April 2017, to confront the Amazon Warrior - 60 nautical miles out to sea.

Norman and Howell leapt from their boat and swam in front of the Amazon, preventing it from performing seismic work on the New Zealand seabed.

Norman said he was given no choice but to confront the boat as it searched for fossil fuels.

"As seas rise and extreme weather events become more frequent, we're facing the reality that if we delay action any longer, the worst impacts of climate change for us and our kids will be unavoidable."

In April this year, the Government announced bringing an end to new offshore oil and gas exploration.

Howell was pleased with the decision and said it was encouraging to see how effective peaceful protest could be.

"I'm proud and humbled that I had the opportunity to contribute to an incredible, people-powered movement that's resulted in an end to new offshore oil and gas exploration permits."

"I needed to take action because all of the life on this planet - in its oceans, mountains, rivers, forests, and cities - is marvellous and brilliant. It is delicately balanced and too special to destroy."

Norman previously said forging ahead with the prosecution of climate activists who had campaigned for such change was "counter-intuitive".

"We were running a campaign to end oil and gas exploration and that campaign has been largely successful. We didn't get everything we wanted but we got most of it, with the Government now saying they won't proceed with new oil and gas exploration permits."