Defence barrister Steven Whybrow accused the police officers as "acting no different to a gang of vigilantes" when they rode up on pushbikes to the 19-year-old and his two friends in Riverside Lane about 2am on August 13 last year and searched his pockets.

Police said they had approached the car and the 19-year-old admitted taking a pill. But it was disputed by the defence that he had admitted taking a pill before the search.

On the teenager's evidence, the officer had rode up to him and shone a torch in his eyes before saying "You're f--king high as shit" and patting down his pockets.

Police found tabs of LSD and pills of MDA in the 19-year-old's possession. He was later charged with two counts of drug trafficking, and four other offences in the alternative.

Police can only perform emergency searches under the territory's drug laws if the officer believes on reasonable grounds that it is necessary.

But the police officer informant had given evidence he only had a suspicion that the man had drugs, based in part on where he was in an alley near nightclubs.

Mr Whybrow, instructed by Kamy Saeedi Law's Taden Kelliher, said there was no basis for the search other than the man's presence in the Civic alley and that the search was clearly illegal or at least improper.

He argued in the ACT Magistrates Court for the evidence following the search to be excluded. "This was simply an unlawful search in circumstances where the best that could be said of police is they were reckless," he said.

Prosecutors said the police had acted in good faith, albeit were misguided as to the level of required belief under that power.

Magistrate Glenn Theakston declined to admit the critical evidence of the seized drugs and the messages found on the man's phone. He said he had strong reservations about the conduct of police.

The magistrate said it was telling that if police had paused to get a warrant over the telephone they might not have been successful.

In his ruling, Mr Theakston said there were competing public policy positions in admitting the evidence.

One, the public interest in admitting evidence in relation to the commission of offences and the prosecution of such crimes. And two, the clear public interest in not encouraging or reinforcing unlawful or improper police behaviour.

Mr Theakston expressed surprise that the police informant was unclear as to the extent of his powers. The magistrate also noted that he could not say the offences were serious.

Following the ruling, prosecutors were forced to offer no evidence in relation to the charges and Mr Theakston dismissed all six.

He ordered the informant pay the teenager's costs.