Western Australian Treasurer Mike Nahan has labelled claims his shareholdings represented a conflict of interest "sheer stupidity", after Labor stepped up its attacks over his investments.

The Opposition has called for an independent inquiry into Dr Nahan's finances in Parliament today, after revelations he owned shares in two gold miners, Telstra and insurance giant QBE.

Opposition Leader Mark McGowan said all four presented a conflict, describing it as a situation worse than the one that saw Dean Nalder stripped of the Finance portfolio late last year.

Labor outlined its conflict of interest allegations today including:

Telstra's numerous business deals with the WA Government, involving a mobile phone tower installation program which Labor said Dr Nahan was involved in announcing

Telstra's numerous business deals with the WA Government, involving a mobile phone tower installation program which Labor said Dr Nahan was involved in announcing Arrangements between QBE and the State Government and Dr Nahan's role as the overseer of the WA Insurance Commission

Arrangements between QBE and the State Government and Dr Nahan's role as the overseer of the WA Insurance Commission Local connections of the two gold miners, Medusa Mining and ABM Resources, and possible flow-on effects to those companies of the ongoing review of WA gold royalties

Yesterday, Dr Nahan denied either Medusa or ABM had operations in WA, but it has since emerged one has an office in Perth, while the other planned to process ore mined in the Northern Territory in WA.

Dr Nahan said late today he would sell his shares in the four companies, but hit back at Labor's claims they represented a conflict of interest, describing some of the allegations as "laughable".

"What these people are doing is just throwing muck," he told Parliament.

"I'm being accused of this 'secret strategy' where some years ago I bought shares in Medusa, which has a single mine in the Philippines, [and] then come in to Government and jack up royalties so I can get higher value to offshore shares.

"How do you respond to that sheer stupidity?

"They see a conflict of interest in everything I do but there was not.

"I monitor a possible conflict in everything I do and if there is, I deal with it."

Labor produced a press release sent out in 2014, announcing a $45 million project with Telstra to build mobile phone towers in which Dr Nahan was named and photographed.

The Treasurer said he did not excuse himself from the decision.

Premier Colin Barnett strongly backed Dr Nahan, denying there was a need for an inquiry.

"Mike is a very honourable person and if any issue arose where there was a conflict he would immediately deal with that," Mr Barnett said.

"I always prefer ministers don't hold shares but I'm satisfied there is no conflict of interest.

"We're a small state and you can find potential or make up claims of conflicts about anyone."