The NSW corruption watchdog is being investigated over delays and potential mismanagement in its long-running probe into former premier Gladys Berejiklian.

An Independent Commission Against Corruption report into Ms Berejiklian’s conduct while in a secret, five-year relationship with fellow Liberal MP Daryl Maguire took more than 600 days to be handed down after the final public hearing into the matter. 

It found Ms Berejiklian engaged in “serious corrupt conduct” by breaching public trust and refusing to report her then-lover, but it stopped short of recommending criminal charges against the former premier.

An official appointed to hold the watchdog to account has started an investigation into why the report, which was delivered late last month, took so long to deliver.

ICAC inspector Gail Furness wrote to senior Liberal and former NSW treasurer Matt Kean to advise him of the investigation after he wrote a letter complaining about the delays last month.

In a statement, the inspector’s office confirmed Ms Furness was investigating whether the length of time taken between the hearings ending in September 2021 and the report being delivered in June 2023 amounted to maladministration.

She will also probe the appropriateness of the watchdog’s procedures, including inviting media into the room when the report was handed down.

Her report will be provided to the NSW parliament when complete with the recommendation that it be made public.

Mr Kean said many people shared his belief that the ICAC investigation and findings were turned into “an undignified circus”.

“Now we’ll find out if they’ve also crossed the legal line,” he said.

The ICAC investigation probed whether Ms Berejiklian’s relationship with Mr Maguire, the former MP for Wagga Wagga, influenced her conduct when she was treasurer and premier.

It found Ms Berejiklian breached the public trust in 2016 and 2017 by overseeing the awarding of a grant to the Australian Clay Target Association in Wagga Wagga without disclosing her personal relationship with Mr Maguire.

Delays in the delivery of the 688-page report were criticised by both sides of politics.

Premier Chris Minns previously said public officials were being asked to effectively put their lives on hold for years while an inquiry was finalised and that was “just too long”.

Mr Kean has repeatedly defended his friend and former colleague, earlier slamming the findings as a “ridiculous display”.

“So it has taken ICAC two years to tell us that Gladys Berejiklian has not broken the law,” he said after the report was delivered.

AAP has contacted ICAC for comment.