NSW Multicultural Affairs Minister Ray Williams has been accused of politically interfering in a water investigation, after an independent report revealed he repeatedly lobbied his own government for one of his constituents to be given a free water licence and favourable treatment over an illegal dam.

Mr Williams wrote six letters to the Minister for Water between 2014 and 2017 on behalf on a constituent, Garry Bugeja, who was being investigated by water compliance officers over an illegal dam on his property at Glossodia in north-west Sydney.

The details of Mr Williams' level of intervention into the matter - which included attending a site visit on Mr Bugeja's property with senior water officials - are contained in an Ombudsman's report into water compliance, tabled in NSW Parliament on Friday.

Ray Williams continued to write letters even after he ceased being the local member Credit:Wolter Peeters

Neither Mr Williams nor Mr Bugeja are named in the report, but the case was first revealed publicly in the media last year.