NSW opposition leader John Robertson says he rejected a multimillion dollar bribe from murdered debt collector Michael McGurk.

But the state government has called on him to explain why he didn't report the offer to the Independent Commission Against Corruption (ICAC).

Mr McGurk offered the $3 million bribe to Mr Robertson when he was the head of Unions NSW, News Corp reported on Saturday.

The former debt collector and businessman wanted to buy the union-owned land at Currawong on the northern beaches.

Mr Robertson said he was shocked to have been made the offer.

"I immediately rejected the offer outright and made it absolutely clear that all offers had to go through the tender process and therefore I was satisfied that was the end of the matter," he said in a statement on Saturday.

"The bid in question was rejected by Unions NSW following the tender process."

But NSW planning minister Brad Hazzard told ABC radio: "As leader of the political party now, he really does need to explain why he would have found that acceptable not to report a very serious criminal offer to the police or ICAC."

Eco Villages Australia Pty Ltd eventually bought the union-owned land. Warwick Watkins, former chief executive of the Lands and Property Management Authority, then bought the Currawong land site from Eco Villages in 2011 for $12.2 million without then-premier Kristina Keneally's full approval.

The purchase prompted an ICAC inquiry, which found former NSW lands minister Tony Kelly engaged in corrupt conduct when he backdated a letter used to claim authority to buy the property two weeks before the March 2011 election.

The DPP advised there was not enough evidence for a successful prosecution against Mr Kelly.

Mr Watkins was charged for using a false document and attempting to mislead ICAC.

Mr McGurk was shot at point blank range outside his Cremorne home in Sydney's north shore in September 2009.

Originally published as NSW opposition 'shocked' at bribe offer