Mr Joyce said politics is a "brutal profession" and he knows it more than most.

"It is very sad to lose such an outstanding individual, whose decency has been evident since she was first elected to the seat of Willoughby," he said.

"There are many reasons for her resignation, but one of the most prominent is to clear the air for her colleagues."

Ms Berejiklian resigned at a press conference after the Independent Commission Against Corruption (ICAC) revealed it will launch an investigation related to the former member for Wagga Wagga Daryl Maguire.

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In a statement, Ms Berejiklian said the issues ICAC is investigating are historic matters that had "already been the subject of numerous attacks on me by political opponents during the last 12 months".

"Many of the matters were the subject of questions I was asked by the opposition while appearing before an estimates committee hearing earlier this year," she said.

"I want to be very clear, in all the decisions I have ever made as a Minister or as Premier for New South Wales, my first consideration has always been the well-being and welfare of the people of this state.

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

Mr Joyce offered the former premier his condolences and said she had done an exceptional job serving and leading the state.

"As the New South Wales Liberal Party prepares to elect a new leader, I am certain the New South Wales Liberal and Nationals Government will carry on with the vital work they were elected to do and take the strongest economy in our nation further yet again," he said.

ICAC said in a statement today it will investigate whether Ms Berejiklian engaged in conduct that "involved a breach of public trust" during her secret relationship with Mr Maguire.

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