Three more West Australians have been diagnosed with coronavirus, bringing the state tally to nine.

The two men and one woman from Perth had all recently returned from overseas holidays.

They separately travelled to Europe, the UK and the US.

All three are stable and currently self-isolating at home, Chief Health Officer Andrew Robertson said on Wednesday.

In the video below: Hundreds tested for COVID-19 at Perth clinics

Play Video Hundreds of people were tested for coronavirus at three pop up clinics in Perth Hundreds of people were tested for coronavirus at three pop up clinics in Perth

Dr Robertson said some had developed symptoms after returning and others while in transit.

It brings the number of cases in the state to nine and the tally nationwide to more than 110.

Schools, businesses facing closure

It comes as Health Minister Roger Cook and Premier Mark McGowan released an update to its Pandemic Plan - last updated in 2014.

The guidelines outline how the state will continue to deliver essential services while dealing with the virus outbreak.

It includes preparations to close schools and businesses, cancelling major events and special arrangements for shopping hours.

WA Premier Mark McGowan. Credit: AAP

McGowan said the state government was also looking at payroll tax relief to help the economy.

He urged the community to remain calm, continue to visit public places such as shops and restaurants and listen to health advice.

Cook said that specialist clinics saw 800 people on Tuesday and up to 450 would be tested.

- with AAP