Authorities had earlier warned there could still be cases lurking from a Victorian hotel quarantine worker infected with the mutant UK variant of the infection.

Genomic sequencing has confirmed the 26-year-old Melbourne man contracted the highly-infectious B117 strain, first identified in the UK.

"We've always worked on the assumption that it was going to be this variant," Victoria's Chief Health Officer Brett Sutton told reporters on Friday, noting four of the six residents at the hotel had it.

The man, from Noble Park in Melbourne southeast, tested positive to COVID-19 on Wednesday, five days after completing his last shift at the Grand Hyatt hotel.

The hotel was one of three being used to quarantine Australian Open tennis players, officials and support staff.

Further analysis of the genomics information will be done to determine who the man contracted the virus from.

But Professor Sutton stressed there has been no apparent breach in infection prevention and control protocols.

There were 14,612 test results processed in the 24 hours to Friday morning, with a further 8000 received since. All have come back negative.

Among those to receive a negative result are 16 close contacts of the worker.

"These are good signs ... that we caught this in good time," Mr Andrews said ahead of Friday's national cabinet meeting.

"Whilst there's still days that will be relevant to us, these are, in some respects, the very best outcomes that we could have hoped for.".

Given the man's UK strain diagnosis, Prof Sutton was "a little bit surprised" none among the cohort had returned a positive result but flagged it was still early in the potential incubation period.

"It is a highly transmissible virus. More transmissible than your standard coronavirus before these variants emerged," he said.

"But there is some time to play out and we have tested people relatively early."

A further 1200 secondary contacts have also been identified, including 507 tennis players, officials and support staff.

The Open on Friday afternoon confirmed the entire tennis contingent has returned negative results, paving the way for lead-up matches to resume and the tournament proper to start on Monday.

It comes as it was revealed hotel quarantine organisers held a party to thank those involved in the Australian Open program just days before the worker tested positive.

COVID-19 Quarantine Victoria has confirmed it held a "small function" on Sunday for 60 staff at the View Hotel, another of the three venues used to quarantine Open arrivals.

Three days later the resident support officer tested positive, ending the state's 28-day run without community transmission.

"Staff and guests observed the same public health safety requirements that all Victorians are required to adhere to when attending public gatherings," a government spokeswoman said in a statement to AAP on Friday.

The man was not at the function.

Victorian authorities are also exploring testing hotel quarantine workers on days off after the man's case was not picked up as part of the state's daily testing regime.

Instead, he went to be tested of his own accord after developing symptoms.

"It's a tricky thing," Prof Sutton said.

"There are issues around how that happens and what entitlements they have as workers on their day off."

Meanwhile, Victoria has moved to reclassify Perth and the WA's South West and Peel regions from a "red" to "orange" risk zone, coinciding with the end of its five-day lockdown.

From 9pm AEDT on Friday, those in the area will be able to apply for a permit to travel to Victoria on the condition they get tested and isolate until they receive a negative result.

Victorians are asked to get tested if they have any symptoms, check exposure sites, stay isolated for 14 days if they are a primary close contact, and follow all directions.

Australian Associated Press