Two construction union whistleblowers who appeared on 7.30 last year to allege endemic organised criminal infiltration of the union have been sacked for going public.

Brian Miller and Andrew Quirk, two New South Wales branch officials of the Construction, Forestry, Mining, and Energy Union (CFMEU), were sacked for "gross misbehaviour" via letters sent on April 20 this year.

The ABC's 7.30 program has obtained copies of the two letters.

"Mr Quirk appeared on the ABC 7.30 program without authorisation of the union and purported to speak as a union officer about matters relating to the union," the letter sacking him reads.

"During that appearance he made comments which were false and/or adverse to the union."

In 2013, Mr Quirk became concerned about what he believed were improper relationships between a group of Sydney crime figures including outlaw bikers and construction industry identity George Alex and senior construction union members.

"There have been reports of corruption, association with murderers, association with gangsters, association with terrorists, money being paid to union officials, union officials intimidating other union officials, union officials being forced out of their jobs and their careers, and the silence is deafening," he told 7.30.

In a written statement, a CFMEU spokesperson reiterated the two men were removed due to "gross misconduct".

"They were provided with natural justice in that [dismissal] process," the spokesperson said.

"They have rights to appeal under the rules of the union, which they have chosen not to exercise.

"Both Mr Quirk and Mr Miller have commenced legal proceedings against the union and these proceedings are being defended."

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Pair ostracised after speaking out

Fellow union whistleblower Brian Fitzpatrick, who left the union last year after receiving a death threat, said the dismissals are an attempt to silence the union's critics.

"There's been a campaign against Andrew Quirk and Jock Miller ... because [the union] didn't like what they were saying.

"Andrew went to great lengths in NSW and nationally to tell the union about his concerns, but he was dismissed out of hand."

In October, Mr Quirk and Mr Miller, both long-serving officials, appeared on 7.30 to expose what they allege was serious corruption in and around the union.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 4 minutes 29 seconds 4 m 29 s Watch Dylan Welch's report for 7.30

The pair said they were ostracised as a result.

"Bit by bit, it overwhelms your life, it consumes you and this is all taking place against the backdrop of, you know, going to work every day and dealing with people at your workplace who are pretty experienced thugs, who are plainly sizing you up to see which leg they want to break first," Mr Quirk told 7.30.

Mr Fitzpatrick said it was particularly frustrating that the pair had been sacked for "gross misbehaviour" when that was exactly what they were trying to blow the whistle about.

"It's just a great bloody irony," he said.

"It's an excuse to get rid of them because they couldn't be intimidated into silence.

"Jock Miller had 30 years of service with the union, just a model employee, and of course Andrew had been a delegate for years, an official, [he] supported the union 100 per cent."

Federal Employment Minister Eric Abetz described the sackings as a "sick reflection on the CFMEU".

"That a union that is willing to do deals with the likes of George Alex, with the information that is now being exposed at the royal commission about bribes being paid ... and here we have whistleblowers being put before the kangaroo court of the CFMEU," Senator Abetz said.

"This just goes to show the moral bankruptcy of the CFMEU and yet it is being supported by Dave Noonan, Michael O'Connor, the ACTU and Mr [Bill] Shorten."