People who have had COVID-19 while in hotel quarantine in Perth will now be tested the day before leaving, even if they have already returned a negative result, under new changes requested by the WA Premier.

The change was made after a man who tested negative on day 10 of quarantine, went on to return a "moderately positive" result after leaving hotel quarantine.

The man's employer required him to be retested for the virus.

WA Premier Mark McGowan said he was surprised to learn previously positive COVID-19 cases weren't necessarily required to undertake a test upon leaving hotel quarantine.

"I learnt about that yesterday," he said.

"I requested that there be a test on day 13 before the person is released so we get up-to -date results before someone in this situation is released.

"That was a surprise to me."

WA Health said previously, all cases were considered cleared of their infection based on the national guidelines, which does not always involve a test prior to completing quarantine.

But a pre-clearance test will now be performed on all cases.

Other guests in hotel quarantine must be tested on day one, five and 13.

More to come.