The Independent Broad-based Anti-corruption Commission tabled its Operation Richmond special report in Victorian parliament on Wednesday after bids to prevent its publication.
It examined allegations of corrupt conduct during enterprise bargaining agreement negotiations between the Andrews Labor government and the United Firefighters Union (UFU) in 2016.
IBAC opened the investigation in November 2018 following complaints the union's secretary Peter Marshall blackmailed or compromised then-premier Daniel Andrews to change his position on the agreement.
The corruption watchdog found no evidence to support those claims or any evidence of corrupt conduct under Victorian law.
But it found senior officials and public servants acted contrary to government policies, procedures and conventions, increasing the risk of corrupt conduct.
"While IBAC did not find that any individuals engaged in corrupt conduct as defined in the IBAC Act, IBAC did find serious and concerning conduct by senior public officers," the report reads.
One instance concerned four days of undisclosed negotiations between former special minister of state Gavin Jennings and Mr Marshall in 2016.
Authorised by Mr Andrews, those meetings occurred in rented meeting spaces and a hotel without the knowledge of the then-responsible minister Jane Garrett, the Country Fire Authority (CFA) or government agencies.
IBAC found those involved in the meetings were instructed to keep them confidential and concluded with a handshake deal.
The government later presented the deal to cabinet as a "UFU proposal", without informing the fire agencies or members of government of the prior negotiations.
The watchdog claimed the secret deal weakened the government's bargaining position and found Mr Marshall expected the deal had to be honoured.
He resisted any significant changes and threatened industrial action or public disclosure of the undisclosed negotiations, it claimed.
"The government placed itself in a position where Mr Marshall was able to exert extraordinary influence over the negotiations," the report reads.
"The actions of the premier, minister Jennings and (then Department of Premier and Cabinet deputy secretary Tony Bates) over this period were contrary to government policy, procedures, convention and their respective codes of conduct."
IBAC also found Mr Marshall, who later fought to prevent the special report from being published, was given privileged access to senior ministers not available to other stakeholders.
The probe also investigated the amalgamation of the CFA and Metropolitan Fire Brigade, leading to the formation of Fire Rescue Victoria in 2020.
It did not substantiate any allegations of corrupt conduct, but did find Mr Marshall sought to influence the appointment of the state's first Fire Rescue Commissioner.
It also found MFB board member Greg Sword shared confidential information with him in 2019, in breach of his professional obligations.
IBAC made three recommendations, including to strengthen integrity standards for ministers, introduce a ministerial handbook and improve training on minister's obligations.
Commissioner Victoria Elliott said the report showed how leaders could undermine accountability when they failed to act with integrity.
"The matters investigated in Operation Richmond may have occurred some time ago, but the corruption risks and vulnerabilities highlighted ... are serious and persist in Victoria's public sector," she said.
Premier Ben Carroll, who has served in the state Labor government's cabinet since 2017, said he was yet to read the report but committed to implementing its recommendations.
"These recommendations go to integrity, I did say integrity is not negotiable," he told reporters.
"I can't answer for the former premier or former ministers. What I can answer for is what we do from here."
