Former WA treasurer Troy Buswell's Busselton electorate office in Busselton has been locked down after police received a report of an envelope leaking white powder.

Police were alerted to the suspicious package at about 5:45pm yesterday after staff found it while sorting incoming mail.

The building was evacuated and police sealed off the Prince Street office.

A specialist chemical analysis team has been sent to the premises, Inspector Derek Staats said.

Five suspicious packages, which were found to contain flour, have been discovered in the past week, including a number addressed to high-profile politicians.

In a statement, WA Police spokesman Dominic Wood said tests will be done.

"It's too early to confirm the details of any written content, but it is likely that this is linked to similar letters addressed to the electoral offices of the Premier and Treasurer, Parliament House, the West Australian and the Taxi Board last week," Mr Wood said.

Packages addressed to Treasurer Mike Nahan, Premier Colin Barnett, the editor of The West Australian and the Taxi board were discovered on Wednesday.

A fifth was discovered Thursday at Parliament house, also addressed to the Premier.

Security has been heightened around WA ministers following the discovery of the envelopes.

The previous envelopes had threats printed on them.

Following the discovery of the earlier envelopes, WA Police Commissioner Karl O'Callaghan said even though the powder was found to be flour the incidents were being taken seriously as they represented a threat against senior people in Government.

Mr Buswell could not be contacted for comment.

He has remained out of the public eye since stepping down as treasurer and later being charged with a series of driving offences allegedly committed in February.