The arm of the Labor Party once stricken by the so-called NSW disease may suddenly be enjoying a rosier prognosis, but it is not kicking up its heels yet.

Opposition MPs were cautious on Wednesday over whether Premier Barry O'Farrell's downfall would trigger a spike in support for Labor, whose primary vote hit a low of 25.6 per cent three years ago.

Labor leader John Robertson on Wednesday claimed the parliamentary wing had turned over a new leaf since its scandal-ridden days in government. "I heard the message in 2011 from the people of NSW and I have spent the last three years out there rebuilding trust with the people and the Labor Party," he said.

"The people of NSW . . . were promised an end to the scandal, a government that would be whiter than white . . . but we don't see that occurring."

Senior Labor MP Michael Daley said the government's huge majority meant the opposition's job remained formidable.

"If [Mr O'Farrell's resignation] shows anything, it shows the unpredictability of politics," Mr Daley said. "It's crazy to be trying to predict what might happen in a week let alone a year. For us, the task is just as difficult as it's always been.

"This has been a very bad government from the start, characterised by Liberal Party mates, donors and lobbyists running the state. "

But for a party that seemed destined for two, if not three terms on the opposition benches, the playing field has suddenly levelled.

Labor won the Miranda byelection last October with a massive swing against the government.

Then a Fairfax-Nielsen poll last month found voters were threatening to throw out the O'Farrell government after just one term. It showed Labor was leading the Coalition 51 per cent to 49 per cent on a two-party preferred basis and it stood to regain up to 25 seats lost to the Coalition in 2011.

Observers have noted the contrast between a Liberal premier falling on his sword over a purported memory lapse, and the former Labor MPs Eddie Obeid and Ian Macdonald, who have suffered few repercussions since being found corrupt by the Independent Commission Against Corruption.

Mr Robertson's past conduct is also under scrutiny. His role in the sale of former union retreat Currawong is being investigated by a parliamentary inquiry after he admitted he was offered a $3 million bribe during the tender process. It will examine Mr Robertson's involvement in the sale when he was head of Unions NSW in 2007.

Mr Robertson said the corrupt MPs had been expelled from the Labor Party and he derided the inquiry as a "kangaroo court".

There may be further shocks to the Liberal Party in the next few weeks when former energy minister Chris Hartcher and central coast MPs Chris Spence and Darren Webber appear before the ICAC over allegations they "corruptly solicited, received and concealed payments". They have been suspended from the party.

Mr Robertson pointed to Labor's manifesto launched last year, called "A New Standard". Among other reforms it requires frontbench Labor MPs to divulge their taxable income for the first time, and record the pecuniary interests of spouses and dependent children. He wants the rules applied to all MPs.