Newcastle MP Tim Crakanthorp has been forced to resign from state cabinet after failing to disclose "substantial private family holdings in the Hunter".
Premier Chris Minns said late Wednesday afternoon that Mr Crakanthorp, the newly appointed Minister for the Hunter and Minister for Skills, TAFE and Tertiary Education, had been referred to the Independent Commission Against Corruption.
"I have formed the view that Mr Crakanthorp has failed to comply with his obligations under the ministerial code of conduct to promptly notify me of a potential conflict of interest," he said.
"This has given rise to concerns that he may have acted in matters in which he had a conflict between his public duties and the property interests of members of his family.
"This is unacceptable and I have referred the matter to the ICAC."
Mr Crakanthorp said on Wednesday night that he had self-reported an "unintended omission from my ministerial disclosures".
"In doing so, I identified a potential and accidental breach of the ministerial code of conduct," he said.
"This was an unintended breach, and I self-reported this to the NSW Premier.
"I appreciate the high standards ministers must be held to; this is why I self-reported this breach.
"I have never acted in an inappropriate manner."
Mr Minns said Mr Crakanthorp's failure to comply with the "letter and the spirit" of the ministerial code had "caused me to lose confidence in his ability to discharge his duties as a minister".
"In light of this, I have asked Mr Crakanthorp to resign and he has agreed."
Mr Minns said he had learned earlier in the day that Mr Crakanthorp's family held commercial real estate holdings in a "clear" breach of Labor's promise to clean up NSW politics.
"I formed the view that minister Crakanthorp failed to comply with his obligations under the ministerial code of conduct and he failed to do that promptly, providing potential conflicts of interest to the NSW cabinet office and, through them, to me as the premier of NSW.
"This has also given rise to concerns that he may have acted in matters in which he had a conflict between his public duties and private interests of members of his family.
"In short, minister Crakanthorp did not supply information about substantial private family holdings in the Hunter region until recently when he should have, constituting a clear breach of the ministerial code.
"I have forwarded that information and other information to the Independent Commission Against Corruption for an inquiry by them if they choose to do so."
Mr Crakanthorp will continue as the Member for Newcastle, but Swansea MP Yasmin Catley will take over as Minister for the Hunter.
Mr Minns said the Labor government had been elected "not long ago with a promise to restore trust and integrity to public matters".
Mr Crakanthorp, a former Newcastle councillor, was elected in 2014 in a by-election and served eight years on the opposition back-benches before being elevated to the Minns ministry in April.
His father-in-law, Joseph Manitta, has been a property developer with substantial land holdings in the Hunter, but it is unknown if the alleged breach of the ministerial code relates to Mr Manitta's property.
The Newcastle Herald reported in 2014 that Mr Crakanthorp's parents-in-law owned an 8000-square-metre development site at Wickham which would likely need to be bought by the government if a planned Stewart Avenue bypass went ahead.
Mr Crakanthorp received support on Wednesday from Hunter-based Nationals senator Ross Cadell.
"I have always respected his commitment to the city of Newcastle and its people, a dedication that never faltered," he said.
"Today's revelations from the Premier concerning land holdings by the family of Mr Crackanthorp's wife come as no surprise to me as they are openly known.
"I would be very surprised if this was anything more than a clerical oversight."