The NSW corruption watchdog has begun an investigation into ousted state Labor minister Tim Crakanthorp over his failure to declare his family’s commercial property interests.

Premier Chris Minns referred the Newcastle MP to the Independent Commission Against Corruption (ICAC) last month after it was revealed he did not disclose family holdings in Broadmeadow while minister for the broader Hunter region.

Documents provided to the NSW parliament reveal ICAC chief commissioner John Hatzistergos confirmed the anti-corruption body was conducting a “preliminary investigation” into whether Mr Crakanthorp breached the ministerial code of conduct.

But the ICAC and the state’s Cabinet Office have refused to release further documents to the parliament’s upper house, claiming their disclosure could prejudice the corruption probe.

Those documents include briefings prepared for Mr Minns, as well as disclosures made and information sought by Mr Crakanthorp in relation to the ministerial code.

The minister was forced to resign from his portfolio after disclosing that his wife and her family had substantial property interests in Broadmeadow, which is home to major stadiums and is on the proposed route for the Newcastle light-rail extension.

While his portfolio had no executive powers, Mr Crakanthorp was responsible for lobbying and co-ordinating other ministers to drive development in the Hunter.

The Newcastle MP, who remains on the back bench, said he self-reported the breach after becoming aware of his in-laws’ holdings and the potential conflict of interest.

Mr Minns previously described the earlier non-disclosure as a significant breach of the ministerial code.

– AAP