The email released under LGOIMA shows Greenpeace alerted Mr Belford, and Mr Beaven to this campaign with directions on how to filter the emails days beforehand.
Mr Butler said he was disgusted elected officials were involved in trying to "bring down" other councillors, so they could not go about their business.
"They knew about it [Greenpeace's email campaign] and they could have stopped it," he said. "I ask for both their resignations forthwith."
Affected regional councillors have expressed disappointment it appears their colleagues were involved in the campaign, which say they was a form of "harassment".
Councillor Debbie Hewitt - who had to change her email address as a result of the 8000 to 10,000 emails she received - said it was a disgrace "that colleagues you work with would stoop to those low levels".
Councillor Alan Dick said the exchange "clearly indicates they were complicit in the email blitz attack, which was a most unethical way of trying to influence people's opinions and voters".
He said it was shameful some councillors had colluded with Greenpeace - whose behaviour he labelled "ecological jihadists" - in this "unsavoury and unprofessional attempt to overthrow and interfere with [council] process".
There could be a move at the next council meeting, he said, "looking for some consequences". He did not want to speculate on what these might be.
Former council chairman Fenton Wilson said he did not know what sort of action could be taken, given nearly a year had passed.
"But looking at the standing orders, Peter Beaven has demonstrated this type of behaviour and now he's a director of our investment company. He wasn't acting in the best interests of the council in being complicit in this event, it's as simple as that really."
However the pair have labelled the implication they were involved in the campaign as "ridiculous", and "bulls***". Last term they were among the four councillors - the minority - against the dam.
Mr Belford said Greenpeace's campaign "came out of the blue ... as far as I was concerned they initiated it and I found out about it basically when I got that email. I had nothing to do with their planning or initiating this, or any other lobbying tactics."
He had not alerted other councillors as he thought the instructions were sent to a number of them.
"From the tone of the email to me it looked like she was trying to basically to say to folks that she thought were sympathetic to Greenpeace's view that 'Hey, this is about to happen and here's something you can do as a matter of convenience for yourself'.
This was echoed by Mr Beaven. He added: "If it had worked, well I might have ... I had to spend a long time getting them out of my inbox like every other councillor so there wouldn't have been much point in alerting other councillors."
He said he could not remember if he had alerted councillors Rex Graham or Rick Barker about the campaign - the other two councillors against the dam.
Mr Barker could not remember if he had been alerted about the campaign. Mr Graham could not be reached for comment.
Greenpeace agricultural campaigner Genevieve Toop - who sent the email in question - said she thought it had been sent to the four councillors who were against the dam.
"We sent them an email to let them know about the email filter and how to set it up, because we felt they were respective of people's views and they were listening to them. They did still get the emails, they just knew how to filter them."
She also disputed that it was "spam" as the emails were sent by individuals, rather than computer programs.
"It's democracy in the technological age."
Ms Hewitt said if this information had been released last year, it could have changed the future of the Ruataniwha Dam, which now looks bleak.
"If that information had come to light last year then there could well have been a very different election result," she said. "Quite frankly councillor colleagues who stoop to those levels and are involved in that sort of harassment of their colleagues, I don't think they deserved to be elected representatives.
"If that had been sorted out last year people may well have voted ... if you look at bigger issues, Ruataniwha might not have ended up in the mess it's in."
As a result of the November local body election, the majority on the council swung from those in support of the Ruataniwha Water Storage Scheme to those against.
