Labor leader John Robertson displayed an ''error of judgment'' by failing to report an attempted $3 million bribe from late businessman Michael McGurk, an inquiry has found, potentially reigniting questions over his leadership.

The parliamentary inquiry into the sale of the former union retreat Currawong at Pittwater found that the offer, made when Mr Robertson was head of Unions NSW, should have been reported to police and his own organisation.

Former union retreat Currawong, at Pittwater. Credit:Paul Miller

It would also have been ''reasonable and prudent'' for Mr Robertson to have disclosed the offer later, when he was a Labor minister, the report said.

The upper house inquiry was chaired by Shooters and Fishers Party MP Robert Brown, with Greens MP John Kaye as deputy. It also comprised three government MPs, who believed the findings did not go far enough, and two Labor MPs, who described the inquiry as ''a political stunt''.