The new case, a man in his 30s who worked at the Indian Ocean Brewing Company and had a meal at the venue on the same night as the original case, brings the total number of known infections in Perth's current outbreak to three.

Mr McGowan had earlier announced that a 32-year-old woman had become the state's second case in recent days following "minimal contact" with the previously known case, a WA woman who had visited Sydney.

Starting from midnight on Monday local time, residents in the Perth and Peel regions will enter a full lockdown for four days.

Mr McGowan announced the emergency measure on Monday night.

"Any new positive case is not good," he said.

"What is even more concerning is this most recent infection appears to have involved only fleeting contact with the original case.

"That is why based on advice from the chief health officer, effective from midnight tonight, Perth and Peel will enter a full lockdown.

"We're confident if everyone does the right thing, we'll get through this.

"It's currently four days. And hopefully by Saturday we'll be out the other side. But we don't know what's going to happen in the next four days."

Under the lockdown, which will begin from 11.59pm on Monday local time, people will be required to stay at home except for a handful of reasons.

Among those are people who cannot work remotely, with a full list of essential workers and services to be uploaded to the WA Health website, shopping for essential items and those with medical or healthcare needs.

Also allowed are those meeting compassionate requirements and looking after the vulnerable, to get vaccinated, and to exercise with a maximum of two adults from the same household, plus children under-18.

Exercise is limited to one hour per day, within a 5km radius from home, and masks must be worn except for vigorous activity.

Restrictions will remain in place until at least 00.01am on Saturday July 3, with restaurants, cafes, pubs and bars only able to provide a takeaway service and retail permitted only for essential goods and services.

Public venues such as swimming pools, recreation centres, gyms, and places of worship are to remain closed.

Community sport and private outdoor gatherings are not permitted, with weddings restricted to five attendees, and funerals to 10.

Schools, boarding schools, and childcare centres in the Perth and Peel regions will remain open, other than the three schools directly impacted and already listed as potential exposure sites - Connolly Primary, Koondoola Primary, and Burbridge School in Koondoola.

Unless otherwise exempt, people in Perth and Peel regions are required to wear masks outside of the home in all indoor settings, on public transport and outdoors at all times.

Anyone who has travelled outside the Perth and Peel regions since Sunday must continue to wear masks in public.

"Today's developments are very, very disappointing but we can take comfort in the fact we have acted quickly," Mr McGowan said.

"We introduced restrictions including mandatory mask requirements which will hopefully have helped minimise any spread. We have done it before.

"We've listened to the health advice and come together to keep each other safe and to keep our state strong and healthy.

"And now it is time to do it again."

Australian Associated Press