CCTV cameras that could have helped prevent Darwin's office bombing were switched off because police had been using them to perv on schoolgirls.

An internal email sent by Acting Senior Sergeant Brendan Muldoon to senior police on Wednesday said the CCTV cameras monitored at the Darwin Police Station had been switched off while allegations of inappropriate use were investigated.

"It didn't take that long and somebody has already used the CCTV system inappropriately,'' the email seen by the NT News said.

"This occurred at Darwin Police Station where a member/s have used the cameras to check out a group of girls, the footage is very damaging! While this is being investigated, the CCTV system has been turned off at Darwin Police Station.''

The federal and NT governments spent $3.3 million installing the 62 cameras in Darwin and Palmerston.

The cameras - which can pan, tilt and zoom - were switched on less than six weeks ago.

When they were turned on last December, Chief Minister Paul Henderson said they were installed so police could monitor "suspicious activity'' and prevent a crime before it occurred.

But no-one at Darwin Police Station was watching on Wednesday.

Police spokeswoman Katie Fowden said the vision was still being monitored at the Joint Emergency Services Communications Centre in Berrimah.

She said members at Darwin station would "rarely'' use the CCTV equipment "unless there is a particular operation under way''.

"To do so would simply duplicate the work based in the JESCC which is overseen by the Territory Duty Superintendent - including the monitoring of the incident on Wednesday at TIO,'' she said.

Police are now using vision recorded from the CCTV cameras to help piece together what happened before and after the bombing, and last night released vision of a purple Holden Barina they believe was used by the alleged bomber.

Live vision from the cameras will not be available to police at Darwin station until an Ethical and Professional Standards investigation into the inappropriate use is complete.

In his email, Acting Sen-Sgt Muldoon said a swipe card system would now be introduced.

"At the moment we don't have individual sign-ons but this will change shortly (maybe two weeks) where you will use your swipe card to log on to this computer (you will then be able to track who was looking at what),'' he said.

"When we start with the swipe card access I can foresee that some idiot will do this again and find him/herself unemployed.

"At this stage the system will remain at Casuarina and Palmerston as members ... have so far used the system correctly.

"Please remind your members that they are accountable and the system is a very useful investigation tool and not meant to be used for fun or pleasure.''

In a statement sent to the NT News last night Commander Kate Vanderlaan confirmed the matter was being investigated.

"The allegation of inappropriate use relates to approximately 30 seconds,'' she said.

"CCTV is a very important safety and investigative tool for NT Police and any claims of inappropriate use will be taken seriously.''