Labour and the Greens say their support for the Keramadec ocean sanctuary is in the balance, as the dispute between the Government and the Māori Fisheries Trust continues.
The two parties are set to go to court over the proposed 620,000 square kilometre sanctuary because of a lack of consultation with Māori.
Prime Minister John Key surprised many at the United Nations General Assembly in New York last year when he announced the sanctuary.
It would create a no-take zone for fishing and mining, but Māori say they have rights to fish there under a 1992 Treaty of Waitangi settlement known as the Sealord deal.
The Government says Māori haven't fished there for a decade, and fish in the area are migratory and can be caught elsewhere.
The Māori Fisheries Trust compared the lack of consultation to the foreshore and seabed debate of 2004, which plagued then-Prime Minister Helen Clark.
The fallout led to the formation of the Māori Party, which is now considering its future with National as a confidence and supply partner.
On Tuesday, Labour leader Andrew Little said his party has "serious reservations" about the legislation, but is supporting changes to the Bill which would "preserve the legal rights of the Māori interests who are asserting those rights at the moment".
Mr Little says at the "very least" the Bill should include the right to go to court over the Kermadec sanctuary.
"If the Government simply can't get on and achieve a good settlement, we can't guarantee we'll continue to support it."
He says the Government is trying to "ride roughshod" over the 1992 settlement.
"It's got to this position because it did a rush job so John Key could go to the UN last year, get some glory about announcing this great sanctuary - but they just hadn't done the work."
The Greens' support for the sanctuary also has conditions.
"It must address the issues Maori have raised," co-leader Metiria Turei says.
Maori Party co-leader Marama Fox says she has met with Te Ohu Kaimoana and told them they support the trust's position.
"For us, it's about the Treaty right, about any new law not being written over the top of any existing settlement to ensure Treaty rights that have previously settled and those being negotiated are held up."
However, they haven't had a further meeting scheduled with the Government.
Ms Fox says iwi aren't fishing the area because the trust has a "self-imposed rahui (temporary prohibition)" on taking fish in the Kermadecs.
"Everyone wants a sanctuary, how that happens and the way in which it's observed can mean different things," she says.
Deputy Prime Minister Bill English will replace Environment Minister Smith as the Crown's lead negotiator as the ongoing legal stoush continues.