Updated April 12, 2020 22:23:54

Photo: Premier Mark McGowan called the low number of new cases "unambiguously good news". (ABC News: James Carmody)

Western Australia has recorded another three cases of coronavirus in what Premier Mark McGowan says is the lowest daily increase since the state's testing regime began.

It brings WA's total number of cases to 517, with 239 people having recovered from the virus.

All three new cases are West Australians living in the Perth metropolitan area, with two of the patients having travelled on cruise ships in the Mediterranean.

Authorities are still investigating how the third person became infected.

The total number of West Australians who have been infected with COVID-19 stands at 449, with the remaining 68 cases involving overseas passengers and crew from the Artania cruise ship.

Mr McGowan said the low number of new cases was "unambiguously good news" and WA had "successfully flattened the curve".

But he said restrictions designed to limit the spread of the virus would remain in place for some time, indicating that would include WA's hard border closure.

Photo: WA's hard border closure is set to stay in place for at least six months. (ABC Goldfields-Esperance: Jarrod Lucas)

"There's going to be restrictions in place at least for six months in one form or another," he said.

"We've been fighting a battle for the last month and we're obviously doing well, but the war isn't won.

"If you lift the restrictions too early and you get community spread of the illness, it's very difficult to put the genie back in the bottle."

Border restrictions here 'for a long period of time'

Mr McGowan said the state and regional border closures, combined with the international restrictions on people travelling overseas, had been very effective.

WA is closing its borders to the east for the first time in its history. Here's how you might be affected.

"I think the border restrictions are the most important things we've done to ensure that we create a fortress of Western Australia in which we are protected from infection from elsewhere," he said.

"They will certainly remain in place for a long period of time.

"Those things together [have] given Western Australia a lot of hope that we will continue to reduce the rate of infection.

"We will continue to review the measures within Western Australia — certainly not the borders, but the measures within Western Australia — on a monthly basis."

Cafe, restaurant rules may be among first reviewed

The Premier said the first of any potential changes to restrictions would be those which might allow people to return to work.

They included rules surrounding cafes and restaurants.

But he suggested people should not be expecting other restrictions to be relaxed next month, including those limiting public gatherings and travel.

"I don't think people should be that optimistic. When I say tweaks, they would be small changes if anything," he said.

"We've done so well so far. Western Australia has been a leading light of the country and perhaps the entire world and I don't want to see us lose that position.

"I don't want to see people's health outcomes impacted. So whatever we do, we'll be very cautious.

"I'd much rather put in place strong borders than close down more businesses. We want to make sure that we keep people in jobs."

The health advice will guide us: Premier

Mr McGowan said authorities remained cautious and vigilant about the spread of the virus.

The stream of COVID-19 cases in WA has slowed to a trickle in recent days, but there are some big risks if the state limits the outbreak too effectively.

"We know that what we've done has been effective, but we also want to make sure that we don't loosen [measures] in ways that cause any problems to arise," he said.

"The health advice will guide us in relation to all of these matters. I don't want to give people false hope, but at the same time I don't want to be too pessimistic.

"If the medical advice says that there can be some alleviation of restrictions and we have very low numbers of infection, and certainly infection of Western Australians from contacts within the state, that will guide any decisions."

Meanwhile, five cruise ships have travelled past Fremantle after leaving South Australia, with two remaining cruise ships still passing the South West and yet to travel up the coast.

Topics: covid-19, infectious-diseases-other, respiratory-diseases, diseases-and-disorders, health, perth-6000, wa

First posted April 12, 2020 16:08:38