Another suspicious package has been found at WA's Parliament House, after a number were sent yesterday, including to the Premier and the Treasurer's electorate offices.

The latest parcel, the fifth, was discovered this morning in Parliament's mail room, addressed to Premier Colin Barnett.

Police said preliminary testing has indicated the powder inside the package was harmless.

The powder found in earlier packages has been identified as flour but police said they were taking the threats seriously, and security around ministers has been heightened.

The Premier said he was not worried by the threats but he was concerned for his staff and the public.

"They are sinister, they can be very disturbing to people who receive the package," Mr Barnett said.

"My electorate office was basically in lockdown for several hours. It happens, but it seems to be an increasing trend in our society for people to make threats, and there's a huge drain on police resources."

Mr Barnett said police responded quickly and precautions had to be taken.

"I'm annoyed that these sorts of threats are made," he said.

"Often telephone threats are also made; they're disruptive, they're expensive and they can be extremely disturbing or worrying to individuals."

Concerns about copycat offences

Two other packages were found on Wednesday at the mail sorting centre near Perth Airport, forcing an evacuation of the building.

One of them was addressed to the editor of The West Australian newspaper.

Police were stationed outside Premier Colin Barnett's Cottesloe electorate office. ( ABC News )

"The powder has been analysed and turns out to be flour so it's an innocuous substance, but we are taking the threats very seriously and we have a full team of investigators on this," said Police Commissioner Karl O'Callaghan.

"We need to take it very seriously because obviously there is a threat against senior people in Government.

"We don't believe this is targeted against one particular person or any one particular issue but we do believe it's from the same source."

Acting Commander State Crime Tony Flack said threatening elected members of Parliament constituted a serious criminal offence.

"It's an attack on our system of Government," he said.

"It scared the living daylights out of people going about their normal business so it had its ultimate consequence and I won't accept that 'by the way it didn’t reach the politician so it's not as serious'".

He also confirmed police were extremely concerned about copycat behaviour.

Motive behind packages unknown

The packages had printed threats, but nothing directed at any individuals or any policies.

"It's very difficult to work out a motive from the material we've got at the moment so we just can't predict that," Mr O'Callaghan said.

The state security unit is to increase its monitoring and protective measures.

"Our dignitary protection people have reviewed the level of security around the ministers, and they will be changing some of their procedures as part of this," said Mr O'Callaghan.

"We will be reviewing those procedures and meeting with the agencies that are responsible for this to see if there's some way we can tighten this up so it doesn't occur like this in the future."

Ministers have been told to be on the lookout for similar packages, or other kinds of threats.

A sniffer dog and handler walk outside Parliament House after the discovery of a suspicious package. ( ABC News )

Acting Commander Flack has confirmed there was currently no screening of mail going into electorate offices.

"That's something that our State Security Unit will look at in terms of future screening," he said.

"There will be additional costs and processes on establishing screening processes in those electorate offices."

Mr O'Callaghan said police were doing everything to resolve the investigation.

"We have forensic procedures which we'll apply as part of this investigation and we're also asking the public for assistance, so if anybody knows anything about this, we're asking them to phone police or Crime Stoppers," he said.

Police have warned that anyone who sends packages with threats could face heavy penalties.