WA Labor has called for an independent inquiry into Dr Nahan's shareholdings after declaring he made false claims in Parliament while defending ownership of his Telstra shares.
Opposition Leader Mark McGowan said he was concerned Dr Nahan had held shares in companies while the Government had financial, business or regulatory relationships with those companies.
Mr McGowan said Dr Nahan falsely claimed that he had not announced a decision about Telstra despite his name and picture appearing on a joint media statement announcing the deal.
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READ NOWMr Nahan sold a host of his shareholdings in April 2013, but chose not to devolve his mining, insurance and telecommunications interests.
"This is shonky because Dr Nahan ignores the distinction between his private interests and his public duties," Mr McGowan said.
�Dr Nahan admitted he failed to declare his interest in Telstra shares when the Government approved a $45 million deal involving Telstra.
"Selling the shares after being caught out is not good enough. A breach is a breach and there needs to be a consequence."
But Dr Nahan, who was elected in 2008, said there had never been a conflict of interest and it was untrue to claim his share interests had not been disclosed.
He said they had always been registered on the Parliamentary and Ministerial Registers.
He said he could not afford to be distracted from major tasks such as producing a State Budget at a time when the collapse in the iron ore price had left a $5 billion hole in the State's revenue.
"It is for this reason that I have divested my remaining share portfolio, not because there was ever a conflict of interest but because the petty antics of the Labor Party are a distraction that I and the State don't need at this crucial time," Dr Nahan said.