On Sunday night, about 700 National Party billboards across the country were altered by activists who fixed new slogans over the billboards.
The slogans were blue with white writing, and said things such as "The Rich Deserve More'' and "Drill it! Mine It! Sell it!''.
The Greens had previously denied involvement, however, Dr Norman told media this morning that Green Party member Jolyon White, the partner of Dr Norman's executive assistant Anne Hein , co-ordinated the defacing.
Mr White quit the Greens today after a complaint was laid with the party over his actions.
A Green Party spokesman said Ms Hein had been stood down while Parliamentary Services investigated the incident.
Dr Norman said he learnt of Mr White's involvement only after other Green members recognised his voice in a radio interview this morning.
"At that point, because we knew that Mr White was the partner of my EA, we talked to her about it and found out what she knew.''
Mr White was not an active member of the Greens, Dr Norman said, and the networks he had used to mobilise the people involved in stickering the signs were not Green Party ones.
"I don't know whether there were other Green Party members involved, there may have been, but not as Green Party people,'' he said.
"Clearly it is part of the membership agreement that you don't bring the party into disrepute, so we will be talking to people if we find other people involved.''
Dr Norman said Ms Hein had known about the plan for a couple of months but was not involved in planning it or taking any actions on it.
"She should have been aware of the political implications of what her partner was doing given her connection to the Greens.''
Dr Norman said he had apologised to Prime Minister John Key and had offered whatever assistance was necessary to clean up the vandalism.
The Greens have previously been outspoken about dirty campaign tactics, and were the subject of an anonymous smear campaign in 2005, which the Exclusive Brethren was later revealed to be behind.
"We've had dirty tricks played on us, and we make a point of playing clean politics, so it is embarrassing that it was Green Party member who did it,'' Dr Norman said.
"I know how many volunteer hours go into putting up and maintaining billboards, I've done a lot of it myself, and I want to say sorry to the National Party volunteers.''
In Auckland this afternoon, Prime Minister John Key said it was possible National would still refer the matter to police.
"This was very sophisticated. It wasn't just someone who went out there with a spray can and had a go at a couple of billboards in the dead of night,'' Mr Key said.
Many of the billboards had been permanently marked, and Mr Key said National would be taking up Dr Norman's offer of volunteers to clean the billboards.
He believed Dr Norman was genuine when he said he had not known about the campaign.
"But what is somewhat concerning us that the person involved was the partner of his EA and she knew for two months and did not pass it on to Russel Norman,'' he said.
"You really wonder about the motives. There's no room for negative campaigning in New Zealand.''
