New Zealand First leader Winston Peters has written to each National Party MP asking them to suspend the Resource Management Act (RMA) changes going through Parliament.
Former ACT MPs are also leading a campaign against parts of the bill they say will give iwi greater powers, including over consenting.
The New Zealand Centre for Political Research, headed by former MP Muriel Newman, has taken issue with changes made after the bill came back from select committee.
"These agreements with iwi leaders were imposed on the country behind closed doors.
"The secret deals were shaped with no consultation, no publicity, no warning and no published minutes or agendas.
"By giving iwi seats at the council table you will embed the Māori world view and spiritualism into the heart of local government."
Mr Peters appealed to National MPs to delay the passage of the bill and "step back from this separatist abyss".
"This country's very future is on the line and on this matter only you, as National Party members of Parliament, have the power to do what is right by the entire country."
Mr English said any changes were to improve the process.
"There's parts of it where we've worked with the Māori Party and iwi on streamlining the way they participate in RMA processes and we're quite happy with those bits of the bill that don't change the decision-making process.
"They'll streamline what currently can be costly and confusing processes - happy with that."