A senior Canberra barrister says shoddy police work and a “cowboy” culture in the AFP is eroding its effectiveness. 

The guilty rate in the ACT Magistrates Court has dropped from 95 per cent of defendants pleading guilty or being found guilty in the five years before 2019-20, down to 65 per cent and 66 per cent in the latest two years.

The ACT Bar Association says an AFP culture of collecting evidence in improper ways is part of the reason for the decline.

Bar vice-president Jack Pappas says some officers regularly use “dodgy behaviour” to obtain evidence, which leads to cases being thrown out of court.

“What you have is a culture within the AFP, which I don't think is being addressed, of police acting illegally or improperly,” Mr Pappas has told the ABC.

“There are few police who really have a good working knowledge of the Evidence Act … I just don't think they take the time to trouble themselves over it - there's a bit of a cowboy mentality.

“I've seen some appalling instances of police misbehaviour, but I've yet to see a case where there's the slightest hint that the police will be hauled over the coals or taught to do things differently in circumstances where they've effectively mucked up or ruined a prosecution because of their behaviour.”

Mr Pappas said the high-quality work of Canberra's legal aid lawyers, and a police habit of overcharging defendants, have both led to a drop in the conviction rate. 

“When people are overcharged and they're facing a maximum penalty - let's take, for example, 10 years instead of two years - then the incentive to defend the matter is significantly increased,” he said.

But ACT Chief Police Officer Neil Gaughan says there is “no evidence of a systemic issue in ACT Policing or the AFP”.

He said the conviction rate is now being determined by Productivity Commission data that was not used in previous years.

“There is a footnote in that table, which is specific in relation to the ACT, about caution being used to compare data over time, due to the fact that an increased number of traffic offences were dismissed with no evidence to offer,” he said.

“Because there were some issues with speed cameras over that time, a lot of matters that went before the court was dismissed. So the data I think shouldn't be compared.”

The commissioner and the president of the federal police union both denied the lawyer’s claims, saying that if the issues were as widespread as he said, then AFP watchdogs, the ombudsman or the court system would have revealed them.