But an emotional Ms Berejiklian has strongly defended herself, saying she has always acted with the "highest level of integrity".

In an bombshell announcement which will rock NSW and Australian politics, Ms Berejiklian confirmed she would resign as premier and quit parliament after the news of the NSW Independent Commission Against Corruption investigation broke on Friday morning.

The commission will investigate whether Ms Berejiklian's role in the awarding of grant funding to projects in Mr Maguire's electorate amounted to a breach of "public trust", given she was in a secret relationship with him at the time.

It will also examine the possibility that Ms Berejiklian had allowed or encouraged the "occurrence of corrupt conduct" by Mr Maguire, and had failed to exercise her duties to report suspected corrupt conduct.

The probe will focus on grant funding for the Wagga Wagga-based Australian Clay Target Association in 2016-17, when Ms Berejiklian was treasurer.

It will also look at a grant to the Riverina Conservatorium of Music in Wagga Wagga in 2018. Ms Berejiklian was premier at the time.

Ms Berejiklian revealed she had a been in a "close personal relationship" with Mr Maguire during an ICAC hearing into the former MPs conduct last October.

In a statement to the media at 1pm on Friday, Ms Berejiklian said she had no choice but to resign as premier because she couldn't predict how long the investigation would take and the state needed certainty amid the ongoing pandemic response.

But Ms Berejiklian has all but proclaimed her innocence from the historic allegations, which she said he been the subject of attacks from her political opponents over the past 12 months.

"I state categorically, I have always acted with the highest level of integrity," she said.

"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.

In a veiled swipe at the timing of the commission's announcement, Ms Berejiklian said her resignation could not have come at worst time as NSW starts to reopen from lockdown.

"That is the ICAC's prerogative," she said.

"Resigning at this time is against every instinct in my being and something which I do not want to do, I love my job, and serving the community, but I have been given no option following the statement issued.

"To continue as premier would disrupt the state government during a time when our entire attention should be focused on the challenges confronting New South Wales.

"I do not want to be a distraction from what should be the focus of the state government during this pandemic, which is the well-being of our citizens."

In her final message as premier, which came just hours before she was due to join other leaders at a meeting of national cabinet, Ms Berejiklian said serving in the role had been the "greatest privilege of my life".

"Please know that every day I gave it my all and worked as hard as I could to create a better future for our state and its people," she said.

"I truly believe that New South Wales is a place where every person, irrespective of their background and circumstances has the opportunity to be their best, to make a difference.

"So as I address you for the final time as your premier, please stay the course and stay safe."

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