She has resigned as premier and will stand down from parliament after NSW's Independent Commission Against Corruption announced she was under investigation.

Ms Berejiklian's stunning decision to quit on Friday came soon after ICAC revealed it was looking into potential breaches of public trust.

After four-and-a-half years in the top job, she will resign from NSW parliament as soon as a by-election for her north Sydney seat of Willoughby can be held.

The anti-corruption watchdog says it is investigating whether Ms Berejiklian "exercised public functions" in a position of conflict given her secret five-year relationship with ex-MP Daryl Maguire, revealed at ICAC hearings in late 2020.

A wearied Ms Berejiklian said she had "no option" other than to resign, as she could not temporarily step aside and resume her role after the investigation.

The premier said the people of NSW needed certainty regarding who their leader was during these challenging times.

"I cannot predict how long it will take the ICAC to complete this investigation, let alone deliver a report."

She also said her resignation "could not happen at a worse time", as the state prepared to leave COVID-enforced lockdowns in the coming weeks.

However, the 51-year-old again denied any wrongdoing.

"History will demonstrate that I have always executed my duties with the highest degree of integrity for the benefit of the people of NSW, who I have had the privilege to serve," Ms Berejiklian told reporters.

She will remain premier until the Liberal Party elects a new leader, with Treasurer Dominic Perrottet considered a frontrunner.

Mr Perrottet said he would speak with his family and colleagues over the coming days on "how I can best serve the people of NSW".

He said Ms Berejiklian was among NSW's finest premiers.

Planning Minister Rob Stokes will also reportedly seek the leadership, while Environment Minister Matt Kean and Tourism Minister Stuart Ayres are also considered contenders.

An ICAC public inquiry on the Berejiklian matter will be held for about 10 days from October 18, overseen by Assistant Commissioner Ruth McColl SC.

ICAC said Ms Berejiklian would be investigated over grant funding to the Australian Clay Target Association in 2016/17 and grant funding to the Riverina Conservatorium of Music in Wagga Wagga in 2018.

It plans to look into whether Ms Berejiklian's relationship with Mr Maguire, then the Wagga Wagga MP, affected those arrangements and constituted a breach of public trust or partial exercise of official functions.

ICAC will also investigate whether Ms Berejiklian should have "suspected on reasonable grounds" that Mr Maguire may have engaged in corrupt conduct, and thus failed to report him to ICAC as required.

It will look into whether she "was liable to allow or encourage" his conduct.

Mr Maguire is accused of abusing his public office between 2012 and 2018.

The former state MP is alleged to have used his public office and parliamentary resources to improperly gain a benefit for himself or for G8way International, a company he allegedly "effectively controlled".

Mr Maguire quit the government in 2018 after a separate ICAC inquiry heard evidence he sought payments to help broker deals for property developers.

Opposition Leader Chris Minns said he "never doubted Gladys' dedication to NSW" but called on the coalition government to remain focused on the health and economic fallout of the current COVID-19 outbreak.

"The focus must be to work on behalf of the people of NSW first and foremost, even in the midst of a leadership contest," Mr Minns said.

NSW Deputy Premier John Barilaro said Ms Berejiklian's replacement was a matter for the Liberal Party, but thanked the premier for her service amid the 2019-20 bushfire season and the COVID-19 pandemic.

Prime Minister Scott Morrison also lauded the "heroic" qualities of Ms Berejiklian, saying she was a dear friend trusted by the community.

Australian Associated Press