WA Treasurer Mike Nahan committed a "minor" breach of the Ministerial Code of Conduct by failing to draw attention to his shareholdings before a related matter came before Cabinet, WA Premier Colin Barnett says.

Mr Nahan sold his shares in QBE this week after pressure from the Opposition, which raised concerns about a potential conflict of interest.

He had declared his shareholding but did not bring it to the Premier's attention when an issue related to QBE was considered by Cabinet.

Mr Barnett told Fairfax Radio it was a mistake.

"In this case, it was such a minor omission," he said.

"Shouldn't have happened. Mike [Nahan] knows that.

"It was a mistake, It probably didn't occur to him at the time, but it should have been recorded in the minutes that he held shares."

Mr Nahan earlier rubbished the Opposition's suggestion his shareholdings, which included interests in QBE, Telstra, Medusa Mining and ABM Resources, were an issue.

Labor had criticised his decision to hold on to the QBE shares when he became Treasurer last year, considering his portfolio has responsibility for the Insurance Commission of WA (ICWA).

Dr Nahan rejected the criticism, saying there was no conflict of interest because ICWA "has no relationship with QBE and does not compete with QBE" and the Government had no intention of privatising ICWA.

He said he had held the shares for some time, before he became a Minister, and they were declared in both his parliamentary and ministerial declarations.