The outgoing Australian federal police chief has admitted an investigation into who leaked a sensitive security briefing given to the government on potential risks surrounding the medevac legislation never got off the ground, as officers “immediately” saw a limited prospect of prosecution.

Following the February medevac leak, which led to a rebuke from Australia’s spy boss for what he said was a misrepresentation of the advice, the home affairs secretary, Mike Pezzullo referred the matter for investigation.

The AFP confirmed last month it had dropped the medevac leak investigation, but on Tuesday Colvin said it never got past the first hurdle.

But at least two separate investigations into journalists who published sensitive information which embarrassed the government continue, despite wide spread outrage at the attack on press freedoms.

Colvin, who announced he would step down from the his role in September earlier this week, told the ABC the investigation into leaked security advice which found its way into the Australian newspaper at the peak of the government’s attempts to scuttle the medevac bill never began.

“We didn’t actually even launch an investigation,” Colvin told 7.30’s Leigh Sales.

“Every matter that come in of this nature, we evaluate to see what we think the prospects of success were. And in that case, we didn’t actually commence an investigation, because we immediately saw that the prospects of a successful investigation or prosecution were limited and the circumstances were entirely different to those other matters.”

The AFP raided News Corp political editor Annika Smethurst’s home last month, over articles she wrote in April 2018 on the government plans to expand the powers of the nation’s domestic surveillance agency.

The following day, the ABC’s Sydney headquarters were raided over articles known as the “Afghan Files”. Smethurst and the two ABC journalists, Daniel Oakes and Sam Clark remain under investigation, and at risk of prosecution. In the ABC case, police had already charged who they believed to have leaked the documents.

Colvin said those cases were different.

“What I can see is that there’s a lot of reporting on selected matters,” he said.

“We received in the order of 12 to 15 unauthorised disclosure leaks every year. So these, these aren’t rare occurrences and in my term as commissioner, we probably received well over 50 unauthorised disclosure investigations.

“Not all of them lead to search warrants being conducted on media organisations, or journalists, very few do, as you can see.

“And also many of them just don’t go anywhere, because these are inherently difficult investigations for us to prove.

“So people will speculate about motive. What I’m saying is the motive is quite simple – is there evidence available, do we think there are prospects for success.”

Colvin, who had previously raised the growing workload of his police force in Senate estimates said the investigations into the journalists had been raised by “senior members of the government, in this case, secretaries of the departments” and he did not believe the investigations to be a waste of police resources.

“In some cases … they were matters commented upon by members of the parliament, both sides of the chamber, to say what a grave, what a bad unauthorised releasing of information this was, what a grave danger this was, how these were national security classified documents,” he said.

“In the broad scale of things the AFP is responsible for and there are many things, we have to weigh this up, time and time again.”

Colvin said the investigations into both Smethurst and Oakes and Clark were at “a legal impasse” after the ABC and News Corp launched legal action in response to the raids.

“Which means that those investigations will be held in abeyance until we know the outcome of some of those court results and that will determine the next steps.”

He denied claims the raids were designed to intimidate journalists and media organisations.

“I don’t believe that is what we were attempting to do,” he said.

“We needed to elicit evidence to further our investigations.”