The West Australian Government is assessing a cluster of cases of coronavirus in South Australia, less than 48 hours after the state removed its hard border restrictions.

Four people have tested positive for coronavirus in South Australia outside of hotel quarantine.

Anyone arriving at Perth Airport from South Australia will now be tested for COVID-19 either on arrival or within 24 hours of arrival and told to self-quarantine "in a suitable premise" for 14 days.

These arrivals will also be required to be tested on day 11 of their self-quarantine.

This includes passengers from a Qantas flight from Adelaide to Perth that has just arrived in the city.

People who had entered the state by plane were given the option of flying back to Adelaide on Monday after being told of the changes on arrival.

Passengers due to arrive on a flight later on Sunday were told about the new requirements and given the option to disembark.

Anyone arriving in Western Australia by road from South Australia will be told to follow the same self-quarantine and testing rules.

Anyone who arrived in the state from South Australia on Saturday or Sunday will be contacted by WA officials and will be required to be tested for COVID-19 and self-quarantine until results are returned.

The decision comes after an emergency meeting between WA's Police Commissioner, Premier and Chief Health Officer was held late on Sunday afternoon.

This brings South Australian visitors into line with New South Wales and Victoria.

The WA State Government said in a statement: "These are initial steps the State Government is taking to protect all Western Australians, and will be reviewed regularly.

"We will monitor the position in South Australia very closely and will strengthen measures if required.

"We understand these changes will cause frustration for many people, but these steps are being taken on the best health advice to protect everyone."

Earlier on Sunday, Premier Mark McGowan said the state's new controlled border was "very, very safe", although the threat of coronavirus from overseas arrivals remained a significant risk.

WA's hard border transitioned to a controlled border on Saturday, meaning travellers from most states were able to cross the border without needing to quarantine for the first time in 222 days.

As the border opened, a total of 2,087 people arrived in WA via road or air — 449 crossed at the Eucla checkpoint, while 246 entered the state at Kununurra.

Throughout the day, 14 domestic flights arrived, carrying 1,392 passengers.

No passengers failed the health screening or temperature checks, Mr McGowan said, with 87 per cent of passengers having completed their G2G pass declarations before leaving the aircraft.

Another 10 domestic flights are scheduled to land in Perth throughout Sunday, bringing an estimated 1,000 people into WA.

None of the flights are from Melbourne or Sydney, meaning no passengers will be required to quarantine automatically.

Three international flights were also due to arrive.

'Biggest risk' from overseas

WA recorded four new COVID-19 cases overnight — all returned international travellers who remain in hotel quarantine.

Three of the new cases are West Australians and one is from Queensland.

Currently, about 1,025 Australians arrive in WA each week from overseas.

Mr McGowan said that given the risk posed by international travel, the State Government would enact stricter border measures if required.

"If the Commonwealth Government decided to go to alternative arrangements, countries that we think are high-risk, we would not hesitate to put in place much, much harder border controls in the future," he said.

"We don't want to undo all that good work with any country that we think is risky."

That would include a return to "harder border arrangements" with other states, Mr McGowan added.

"It's not about political blame — this is about keeping people safe.

"The premiers are all cautious about these things and I think I'm the most cautious, because Western Australia has done so well.

"We haven't gone down the NSW route of having outbreaks and contact tracing and closing down parts of the economy.

Under WA's controlled border rules, travellers from all states except New South Wales and Victoria may enter WA without special reason and are not required to self-quarantine.

However, travellers must still register their travel arrangements using the G2G PASS application and declare which jurisdictions they have visited in the past 14 days.

Anyone entering WA from NSW or Victoria must self-quarantine for a fortnight and conduct a COVID-19 test on day 11.