Detective Senior Sergeant Ron Iddles, who now heads the powerful police union, has also called for reforms to the way the force protects informers and witnesses, saying they are sometimes abandoned after they give evidence.

A Four Corners and Fairfax Media investigation can also reveal how Victoria Police relocated a second key gangland witness to Europe without telling authorities there of the move.

Furious European authorities only discovered the major criminal was living in their jurisdiction when he died and they found he had multiple identities.

The witness was assisting police with two still unsolved gangland murders. Fairfax Media has chosen not to name the precise location of his death to protect his identity.

The former federal police commander who headed the National Witness Protection Program, Phil Kowalick, said placing a witness overseas without a jurisdiction's knowledge was fraught with danger and could affect Australia's relationship with foreign agencies.

Mr Kowalick said the case is one of many that shows why state and national witness protection regimes need a major overhaul.

The revelations come as Victoria's anti-corruption commission, IBAC, completes a major and critical probe into whether the handling of a key witness by Victoria Police may have compromised the administration of justice.

As part of its probe, IBAC grilled Mr Overland last year, after it summoned him from Tasmania where he heads the state's justice department.

In a statement, Victoria Police has acknowledged previous failings in the management of witnesses and informers, but says major reforms have already been implemented and it is awaiting further recommendations from IBAC

The name of the witness at the centre of the IBAC probe is suppressed but they played a role assisting police in major underworld murder and corruption cases, including the murder of police witness Terry Hodson.

Career criminal Hodson was murdered in 2004 with his wife Christine after agreeing to testify against allegedly corrupt ex-policeman Paul Dale.

Police allege Dale commissioned gangland boss Carl Williams to kill Hodson, but charges against Dale were dropped in 2010 after Williams was bashed to death in prison and Dale denies any involvement.

There is continuing fall-out from the Hodson and Williams murders, with the most recent development being the IBAC probe.

Mr Iddles, along with two detectives who also played key roles in issues linked to the Hodson saga, former senior sergeants Murray Gregor and Charlie Bezzina, have backed the reform of witness and informer management.

Two major questions have arisen from the case of the witness at the centre of the IBAC probe: whether their conduct compromised the administration of justice and whether they were exposed to unnecessary risk by police.

Mr Iddles has revealed he refused an order from Mr Overland to take a statement from the witness because the move would endanger them. "There would have been major ramifications I think for that particular witness," he has told Four Corners.

"I guess I took a stance on my own and decided not to take it [the statement]."  Mr Iddles said the force's desire to solve corruption or other high profile cases often over-rode the need to prioritise "the safety of the witness".

"We all want to have a successful outcome in relation to court cases, but I don't think we do it at the expense of a witness' life," he said.

Mr Iddles has also revealed he recently told retired judge Frank Vincent – who is heading a review of the state's witness protection laws – that further reforms are needed, including measures to better protect witnesses who don't want to join the witness protection program, but who are still at risk.

"We need to look at how are we going to look after that witness. It still could be a relocation, could be security cameras, could be monitoring, could be a whole range of things."  Mr Iddles said the force also needed to better protect witnesses after they testify.

"Sometimes we think, 'well that's it, job's been done, we've got the conviction, let's move on'.  That witness is still at risk for the rest of their life."

Hodson had refused police requests to enter witness protection and the lead detective who recruited Hodson as an informer, Mr Gregor, said more should have been done to protect him.

The detective who initially led the Hodson murder probe, Mr Bezzina, said the force needed "outside organisations looking at us" to improve the very "volatile" process of informer and witness management.