Insider trading by WA government staff following a major contract decision can't be ruled out, the transport minister says, but insists his office wasn't privy to the information before a share price surge.

The Australian Securities Exchange queried NRW Holdings in February before and after it announced it had won the design, construction and maintenance contract for the $2 billion Forrestfield Airport Link, noting its share price had soared.

The contract award was announced on February 18 but the company’s share price moved substantially in the preceding days, rising from 4.9 cents on February 12 to 6.4 cents on February 15.

The corporate regulator, the Australian Securities and Investments Commission, is now investigating possible insider trading among state government staff, taking documents from Mr Nalder’s office and public servants involved in the project’s tender process.

Mr Nalder said he had been told the tender evaluation committee didn’t make its decision until February 16 – the day after the share price spike – while his office was notified on February 17.

“We received that information well and truly after that share price spike so there can be no assertion that anybody in my ministerial office had anything to do with that,” he told parliament.

While Mr Nalder said ASIC had so far not indicated there should be any concern about the conduct of any state government employee, he could not rule out any wrongdoing.

“I cannot stand here and categorically say that someone within the staff has done something right or wrong.

“But what I can assure you is that my office was not informed at the time of that share price spike – there was no flow of information to my office.”

After fellow MPs said it appeared insider trading had indeed taken place, Mr Nalder told reporters he shared their concern, but pointed out parties other than public servants were involved in the tender process.

“It’s too premature to say – it doesn’t look good and so therefore I support the investigation.

“What I don’t support is running off and assuming … that someone within the state government has actually undertaken an illegal activity.”

Mr Nalder said he personally had done nothing wrong.

The opposition, however, said Mr Nalder had downplayed the issue and misled parliament because it appeared he had briefed Premier Colin Barnett on the contract on February 15.