Posters with pictures of plain-clothed officers working on campus and labelled "Narks in our Class?" and "Narkiology 101. How to spot a nark" appeared around the university on Monday.
One poster shows plain-clothed officers involved in the recent arrest of three people at a National Organisation for the Reform of Marijuana Laws (Norml) stand at a Otago University Students Association market day.
The other shows plain-clothed officers at a regular protest "smoke-up" on campus.
The posters name the officers, give their badge numbers and ask people who think there might be an undercover police officer in their class to contact Norml.
Norml leader Abe Gray said he was not sure who had put the posters together and posted them on the Norml website, from where they could be downloaded, but he believed the images were taken from video footage recently posted by Norml members on YouTube.
He said the posters had probably been put together because students felt uncomfortable being under surveillance on campus.
From information police had divulged during various interactions with them, it was believed they were also working undercover in lectures, Mr Gray said.
Dunedin area police commander Inspector Dave Campbell said he was disappointed, but not surprised, photographs of police officers were posted on the Norml website.
Police were running an operation to stop offences against the Misuse of Drugs Act on the university campus and, to date, as a result, had issued nine trespass notices to non-students and three to people enrolled at the university.
Those trespassers included known drug dealers, gang members or associates and one secondary school pupil.
Insp Campbell said trespass notices were issued by police acting as an agent of the university.
The 12 people issued with trespass notices had a total of 26 charges pending before the courts for a variety of drug and other offences.
Plain-clothed and uniformed officers had been used in the operation and a variety of policing methods would continue to be used, Insp Campbell said.
"If people continue to use drugs on campus or incite others to break the law they can expect attention from police."
A university spokesman said no-one was able to comment on plain-clothed officers working on campus until today.
