Police are investigating a fourth suspicious mail package and have cordoned a Riverton shopping centre just hours after the Perth Mail Centre was evacuated.

WA Police spokesman Sam Dinnison said police were investigating a report of a suspicious mail item being delivered to an office at the Riverton Forum in Parkwood and had closed off areas of the shopping centre as officers make inquiries.

It comes after the Tactical Response Group Bomb Response Unit was deployed to the Perth Mail Centre on Boud Avenue, Redcliffe after three suspicious packages were found.

Mr Dinnison said the packages had been deemed "suspicious" because of the nature of the words written on them and confirmed the centre had been evacuated and Dunreath Avenue closed.

Police have not commented about whether the mail item found at Riverton is linked to three other packages that led to the lockdown of the airport mail centre.