Mr Robertson's decision to quit came after senior colleagues made him aware they had planned to visit him at 5pm on Tuesday in a bid to resolve the leadership issue. 

The decision means the ALP will elect a new party leader only three months before the March 2015 state election. The new leader is likely to be chosen by a ballot.

Shadow Treasurer Michael Daley and environment spokesman Luke Foley are being discussed as leadership candidates.

At a media conference flanked by his wife Julie and family members, Mr Robertson - who has been opposition leader since the change of government in 2011 - said he was stepping aside in the interest of party unity.

"It has become clear to me that I have lost the support of some of my senior colleagues," he said.

The push to remove Mr Robertson as leader was sparked on Monday when it emerged that, as the member for Blacktown, in 2011 he signed a letter in relation to a dispute between Monis and his second wife, Noleen Hayson Pal, over visitation rights for their two children.

Mr Robertson defended himself by arguing that it was not a letter of endorsement and that it was forwarded to the department of community services "in a routine manner".

But many MPs questioned Mr Robertson's judgment in signing the letter, given that Monis had two years earlier been charged with writing offensive letters to the families of dead Australian soldiers.

On Tuesday Mr Robertson said as a local MP he had "made a conscious decision that my office would be there to serve everyone in the Blacktown community. However I accept how this may  appear given recent events."

The controversy followed similar unrest within caucus over his voluntary admission to a journalist in late 2013 that he had, while Unions NSW secretary in 2006, been offered a $3 million bribe by murdered businessman Michael McGurk.

Mr Robertson rejected the offer but did not alert the police.

At Tuesday's press conference Mr Robertson did not take questions, nor publicly endorse a successor. 

"Labor must unite behind the leader," he said. "As a life-long Labor man I have chosen to step aside to give Labor this opportunity."

Mr Robertson nominated party reform, including the expulsion of disgraced former state MPs Eddie Obeid and Ian Macdonald, and presiding over the party during the introduction of community preselections as his principal achievement. 

"I accepted the challenge of helping Labor find its best self, the party we all know and respect," he said. "I have rid the party of those who betrayed the great Labor cause.

"We're steadily building a new culture of integrity and corruption resistance, a far cry from the culture I inherited."

Mr Robertson said the highlights of his policy record included support of a second Sydney airport and advocating measures against alcohol-fuelled violence.

The acting leader of the Labor party, Linda Burney, said the state parliamentary party would meet on January 5 to begin the process of electing a new leader.

"John has been a committed and hardworking leader who took on one of the toughest jobs in Australian politics at the lowest ebb for NSW Labor," Ms Burney said. "His reform of the Labor Party, his passion for the people of NSW and his commitment to true public service will not ever be forgotten by myself or Labor."