Tasman district commander Superintendent Grant O'Fee said most of the officers left Nelson for Queenstown on Sunday to work for three days as drivers on the movie.
He said the officers had breached police general instructions by engaging in secondary employment without asking a superior's permission.
Mr O'Fee said he had not known about the venture but if the officers had asked permission he would have granted it.
He said the work was a "one off" and he considered it at the bottom end of the scale of secondary employment because it was only three days' work. It was not a serious breach of police guidelines and he did not plan on disciplining the officers.
Revelations of the Nelson police's involvement follow warnings by Police Minister George Hawkins that there was no room for moonlighting by police staff. Mr Hawkins' office said on Tuesday the incident was a local disciplinary matter and he did not intend taking further action.
Mr O'Fee said representatives from the Lord of the Rings contacted Nelson police at the last minute asking for people to drive trucks and cars to Queenstown.
"I think what has happened is that this request has come through at the last minute asking `Can you help us out?'."
Most of the 15 officers were due to return to Nelson tomorrow. They had taken annual leave and days off to work on the set, Mr O'Fee said.
They will be paid $150 each for the work and be given free accommodation and have all their expenses paid.
"It's more regarded in that they got to see a bit of the country and (the Lord of the Rings) movie and that sort of thing. They weren't expected to be coming back with thousands of dollars." he said.
Mr O'Fee said the movie representatives asked for police help because they were "seen as people who would turn up on time".
New Zealand film producer Peter Jackson has been working on the $360 million movie production of Tolkein's trilogy since 1998. Filming has moved around the country to take advantage of remote spectacular scenery, including in Golden Bay last week.
Besides police involvement, soldiers have also reportedly worked on the movie, including laying soil, tending gardens and building sets.