Western Australia's Parliament has resumed sitting after a cyber security breach caused chaos with the building's communications networks and hindered a number of crucial operations.

Phone and internet networks at Parliament House were shut down after problems began late on Tuesday.

A memo sent to MPs said a Trojan virus had penetrated Parliament's Information Technology network, forcing computers and phones to be isolated.

The breach sparked doubt as to whether Parliament would be able to resume sitting while so many parts of the network remained down, but the Legislative Assembly commenced proceedings at its usual 12:00pm start time.

However, Speaker Michael Sutherland said a number of usual operations would be affected by the network problems.

"The Parliament's IT network is currently offline, which has affected a number of house operations including Hansard publications, the preparation and processing of questions on notice and answers to questions on notice," Mr Sutherland told the chamber.

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"In addition, Parliament House telephones and building paging systems are not operational."

"It is expected that we will be fully operational again by this afternoon."

The telephone network and WA Parliament website were restored a short time later.

Mr Sutherland said the incident had occurred despite Parliament's "sophisticated" firewalls and staff had been working hard to resolve the issue.

It is understood the problem first occurred between 9:00pm and 10:00pm on Tuesday.

Labor Member for Midland, Michelle Roberts, said she had never seen anything like it in more than 20 years.

"I certainly haven't, but it certainly shows that there's a new era in vigilance required, not just on the personal security level, but especially in terms of cyber security," she said.

"I expect there are people who have got access here to all kinds of important information that does need to be secure."

Some MPs had not heard about the breach before arriving at Parliament today.

Labor Corrective Services spokesman Paul Papalia was in the dark about the IT blackout.

He said he had been using a separate government network.

"I hope we're not vulnerable to it because I've been using it this morning," he said.