Health officials say the 13-year-old is a close contact of a previously confirmed case.

It takes Western Australia's total since the start of the pandemic to 662, while the state has three active infections.

From September 27, WA is set to increase its arrivals by 200 per week before adding an extra 300 from October 12.

Premier Mark McGowan said on Friday at least one and possibly two extra quarantine hotels would need to be set up to accommodate the extra 500 returnees.

However a revival of the quarantine program on Rottnest Island isn't expected to be required.

WA has enlisted eight hotels to house people returning from overseas and Victoria, as well as airline staff.

Elective surgery may be impacted by the change to ensure medical support isn't compromised for the increased hotel quarantine efforts.

"I don't want to reduce elective surgery capacity but it may be necessary," Mr McGowan said.

Meanwhile, WA has announced it will start testing wastewater for COVID-19 within the next month.

The analysis aims to identify the existence of the virus to enable more targeted campaigns encouraging people to be tested.

It has been used in other jurisdictions including the ACT and Queensland.

Australian Associated Press