Deep-sea mining is set to go ahead in Tonga after the Pacific Island's government committed to a new agreement.

The revised deal was signed on Monday with Tonga Offshore Mining Ltd, a subsidiary of The Metals Company, a prominent Canadian mining group.

An original agreement was first signed in 2021 amid accusations of a lack of public consultation which drew concerns from civil society, youth advocates and legal experts across Tonga.

The proposed arrangement promises future financial benefits should the proposed deep-sea mining licence proceed past the list of obstacles.

Both the US pathway to mining and the International Seabed Authority – the world's deep-sea mining regulator – say they offer a pathway to mine in international waters.

The Metals Company has been a key player in the Pacific region as it seeks to undertake seabed mining activity under either of the jurisdictions.

But many Tongans remain unconvinced, citing environmental risks, legal liabilities, and a lack of transparency.

A Tongan non-government organisation says the government should not have signed up.

Civil Society Forum of Tonga spokesperson Drew Havea confirmed the organisation had been invited to consult on the proposal from The Metals Company.

Under international oceans law, Tonga has special rights in its own surrounding areas of the Pacific Ocean which possesses deep sea minerals.

The island's existing contract with The Metals Company covered activity in Tonga's block of its area, known as the Clarion Clipperton Zone.

That contract was signed four years ago by then Prime Minister, Pohiva Tu'ionetoa, through the International Seabed Authority framework.

Mr Havea, who has not yet seen the contract with The Metals Company, expected it might provide a path for Tonga to mine through the US pathway instead of the International Seabed Authority.

Just months ago, The Metals Company announced it had signed a new contract with Nauru.

That agreement detailed potential payments to Nauru should mining activity occur through the US pathway.

Nauru, like Tonga, has special rights in the Clarion Clipperton Zone under international oceans law.

Mr Havea said Tonga should use the "current climate" as a way of avoiding deep-sea mining entirely.

Civil Society Forum of Tonga has been a long-standing opponent to Tonga's involvement in the industry.

"We are the sponsor country, and sponsor country will be responsible for any damages to the ocean," Mr Havea told Radio New Zealand.

"It's not the company...that will be responsible, and I think from that perspective, we are quite concerned that we will be liable."

Mr Havea believed The Metals Company was already committed to mining through the US pathway – an observation confirmed by a previous statement.

He said that pathway did not feature in the last deal between the Tongan government and The Metals Company, and therefore the arrangement would have to be set out in a new contract.

"I know The Metals Company has made the application to be part of the US (pathway)," he said.

"So, from Civil Society (members), we hope the Tongan government (doesn't) do anything.

"Because once (The Metals Company is) into the US mining (pathway) that means they are out of the International Seabed Authority, and the contract that we had signed has binded us into the International Seabed Authority."

International environmental lawyer Lori Osmundsen said the existing Tongan government-The Metals Company contract raised a range of legal concerns and risks for Tonga.

Civil Society Forum of Tonga commissioned Ms Osmundsen in 2023 to evaluate Tonga's legislative framework for issues relating to ocean management.

The comprehensive review covered the existing deal between the Tongan government-The Metals Company contract and the circumstances of its signing in 2021.

To this day, the organisation says it had not received an answer on whether the contract in 2021 had been approved by the Tongan cabinet before it was signed, as is required by the nation's laws.

A new sponsorship agreement between the Nauru government and The Metals Company's subsidiary – called the Nauru Ocean Resources Inc – was only signed at the end of May, Ms Osmundsen said.

"That is publicly available – and if that's the recipe for a similar proposed new agreement between Tonga and The Metals Company, it sort of continues on the problems, the legal risks and hazards of the current agreement, and then adds a whole new set," she said.

Ms Osmundsen said the major risk for Tonga in signing a contract with The Metals Company similar to Nauru's deal under the US pathway is it would open Tonga up to being in a commercial-like arrangement which sits outside international laws and would breach the UN Convention over the Law of the Sea.

The International Seabed Authority has announced it is investigating contractors which hold mining exploration permits under its jurisdiction to ensure they have not breached terms of International Seabed Authority contracts, including The Metals Company's partnerships of Tonga and Nauru.

No commercial deep-sea mining has yet to occur, and the International Seabed Authority is still finalising a mining code.

"The contract that resulted (in 2021) is quite disadvantageous for Tonga, and we don't need to repeat that," Ms Osmundsen said.

"So greater scrutiny, greater awareness, availability to Tongan public stakeholders, all of governments to evaluate and weigh in on these incredibly important issues."