AFP hoped Rush might expose kingpins

Australian federal police let Bali Nine mule Scott Rush leave for Indonesia hoping he would lead them to the drug ring's kingpins.

Police have told reporters they knew Rush was linked to a ring planning to import illicit drugs into Australia.

But when Rush and others linked to the syndicate flew out of Australia, bound for Bali, a decade ago, police had an incomplete picture of the operation.

They didn't know the identity of the kingpins or even which drug would be smuggled back into Australia.

They said they lacked sufficient evidence to arrest any of the nine before they left and hoped that letting them go would lead them to the Mr Bigs of the syndicate.

"We needed to find out all that information so we could ... stop further importations occurring and get to the source of the narcotics themselves," said AFP Deputy Commissioner Michael Phelan, who was in charge of the initial investigation.

The AFP has defended its actions after ringleaders Andrew Chan and Myuran Sukumaran were executed in Indonesia last week over their roles in the 2005 heroin smuggling plot.

The AFP denied ever promising Rush's father, Lee Rush, his son would be prevented from leaving Australia or warned he was the subject of police attention.

"It is absolutely not feasible to alert somebody who's going overseas that (they) are of police interest ... in relation to a serious and organised crime investigation of this nature," Mr Phelan said.

The AFP decided to ask Indonesian police to carry out surveillance on the Australians "in the full knowledge that we may well be exposing those individuals to the death penalty".

"We knew what may occur as a result of that," he said.

AFP Commissioner Andrew Colvin said it wasn't true that Lee Rush's tip-off about his son's plans to join the drug ring sparked the police operation.

The AFP was already broadly aware of the ring's activities and Rush was subject to three separate travel alerts based on a range of intelligence.

"I want to take the pressure off Scott Rush's father," the commissioner said.

"The AFP was already aware of, and had commenced investigating, what we believed was a syndicate that was actively recruiting couriers to import narcotics to Australia at the time of Mr Rush's contact with the AFP."

Mr Colvin said the AFP had an obligation to protect Australia from transnational crime and could not limit itself to dealing with countries that shared a similar approach to justice.

"Our strategy for many years is to take that fight offshore whenever possible to minimise and reduce the impact it has here in Australia," he said.

Barrister Bob Myers, who made the initial approach to the AFP on behalf of the Rush family, said the organisation had used the Bali Nine drug mules as bait.

He said the AFP's explanations were nothing new, but he was interested to learn that one Queensland-based officer had taken a stand against exposing Australians to the death penalty.

"He was the only decent one," Mr Myers told AAP.

"The AFP's justification seems to be that they weighed up the lives of the Bali Nine with the lives of all the innocent Australians who were going to be exposed to drugs.

"But I've been saying for 10 years that's not the test. You can't decide to sacrifice the life of any one person even if you realise it's going to save the lives of others. That's not how it works."