New South Wales Opposition Leader John Robertson has effectively admitted he should have reported a $3 million bribe offered to him when he was secretary of Unions NSW.

Mr Robertson told Lateline if he had his time over again he would have reported the offer - made to him by murdered businessman Michael McGurk - to authorities.

"If I found myself in those circumstance now obviously I would do something completely different. Hindsight is a wonderful thing," he told Lateline host Tony Jones.

"If I found myself in those circumstances again I'd report it," he said.

Mr Robertson is set to face a state parliamentary inquiry over his revelation to a newspaper last year that he was offered the bribe by Mr McGurk, who was later shot dead outside his home in Sydney's Cremorne.

Mr Robertson rejected the offer, which related to the sale of valuable union-owned land at Currawong in Sydney's north.

"...let me make it clear that at that time I was sounded out by an individual who seemed to be making an inappropriate offer. I categorically ruled that out," Mr Robertson said.

"That was the end of the matter and that arrangement, those negotiation and discussions went no further. There was no benefit for that individual.

"He (McGurk) appeared to be sounding me out and I made it very clear I wasn't going to entertain anything like that."

Asked if Mr McGurk used the words $3 million, Mr Robertson replied:

"No, he talked about a percentage of what was on offer but I ruled that out."

Asked whether he engaged in a lengthy conversation with Mr McGurk in order to discuss details such as percentages, Mr Robertson said:

"Not long at all. I did what most people would expect and that is ruled it out, sent him on his way. It was pretty much the end of any discussions with them on anything to do with Currawong.

"What I did was what I believe anyone would have done in those circumstances and that is said to this person, 'Not interested, not going to entertain it,' and that is the end of it."

Mr Robertson told Lateline he believed his actions were appropriate at the time.

"I've got nothing to hide. My honesty is not in question at all. What I did is what anyone else would do in the circumstances and made it abundantly clear there was no way I was going to entertain something like that."