National claims former Labour Party president Bob Harvey told Whangamata marina opponents Chris Carter had "the balls" to stop the development days before his final decision.
The Conservation Minister was also soliciting emails to help claim he had public support, the party claims.
National MP Nick Smith disclosed details of an email he said in Parliament was sent by Mr Harvey to fellow marina opponents three days before the decision was announced.
He later conceded an associate of Mr Harvey had sent the email, which refers to messages from Mr Harvey.
Speaker Margaret Wilson accepted a National bid for an urgent debate on the issue, over which the Government is increasingly testy.
National leader Don Brash will meet members of the Whangamata Marina Society today.
Mr Carter's decision to reject the proposed development was despite an Environment Court recommendation to the contrary.
The email was sent by a John Wilson to a range of people.
It said: "We have received confirmation from Bob Harvey ... to say the minister has got the balls to go ahead and stop the marina going ahead at Whanga.
However ... he is getting lobbied real hard and has asked for support from those who will back him making this decision. His neck is on the line."
Recipients were asked to write to Mr Carter, expressing their views.
In another email sent that day, Mr Wilson claims, "Chris Carter is going to pull the plug on the marina, but is asking for us to back him up."
Dr Smith said the email was evidence of "the political machinations that have gone on behind the scenes, the sort of manipulation that Chris Carter and his Labour mates will do to override proper process."
He later described Mr Harvey's apparent knowledge of what Mr Carter was going to do ahead of time a "gross abuse" of the confidential legal process the minister should have followed.
Dr Smith called on the minister to resign.
Mr Carter retaliated by claiming Dr Smith's involvement in the issue as conservation minister during the 1990s was "very murky indeed".
He had made a lot of noise about the fact that the Department of Conservation had not taken part in the Environment Court hearings.
"But what he has not told the people of New Zealand is that DoC was opposed to the marina because of its environmental impacts and lodged an appeal against the Waikato Regional Council's approval of it in 1997.
"It was only after pressure on DoC by its minister that the department withdrew its appeal and settled with the marina society."
He challenged Dr Smith to explain why he pressured DoC "and exactly how he was fulfilling his responsibility [as minister at the time] to the public on whose property this marina will be built".
Mr Carter reiterated he was using his powers as directed by the Resource Management Act.
He was asked by Coromandel MP Sandra Goudie for evidence of his "claim on Breakfast TV, and on radio, that people in Whangamata oppose the marina, when a survey of every household by the Waikato Times shows residents supposed the marina by over three to one?"
Mr Carter replied he had 141 emails, 127 of which were in favour.
His spokesman later presented the Breakfast transcript which showed Mr Carter did not claim he had majority support in Whangamata.
Dr Smith said later, referring to the email, it was "no wonder he has 127 emails in support".
Mr Carter sought leave to table a 1997 DoC paper stating: "The department is opposed to the application. The proposed marina is in an area of salt marsh, which would be destroyed. Written above it in the writing, I believe of the minister of conservation of that time, Nick Smith, is 'Says who?"'
Dr Smith said later he did not believe it was inappropriate to challenge DoC as the appeal was written in his name, as minister, and he was clearly entitled to have input.
The Greens and the Maori Party supported Mr Carter, saying he had followed due process, but United Future joined National in opposition.