Victoria has posted 15 more locally acquired coronavirus cases, including four mystery cases.

The state health department on Friday confirmed 11 of the new cases were linked to known outbreaks, while eight were in quarantine during their infectious periods.

Some 40,737 tests were processed in the 24 hours to Friday morning, while 27,427 Victorians were vaccinated at state-run hubs.

Victorian health authorities are expected to give more details in a briefing later in the day.

However, there are concerns one of the new cases is linked to a special school in Melbourne’s west.

Western Autistic School in Laverton was closed on Friday after a confirmed COVID case of was linked to the bus service used by the school.

Tweet from @VicGovDH

Friday’s developments came as contact tracers try to find the source of four other mystery cases on Thursday.

They included three in two households in Glenroy, one a child who attends Glenroy West Primary School.

Victoria’s COVID-19 Commander Jeroen Weimar said the families did not know each other and their children do not attend the same school or sports club.

COVID-19 has been detected in wastewater in the Glenroy area since August 5.

The other mystery case, a man who lives in Brunswick East, has some social connections to known exposure sites and authorities are confident a definitive link will be found in coming days.

There are now more than 400 exposure sites across Melbourne and its urban fringe, including inner-city tram routes, an Australian Taxation Office building in Moonee Ponds and VicRoads site in Hoppers Crossing.

Seven shops in the South Melbourne Market were listed as tier-one sites overnight after an infected person attended Saturday from 1.30pm. The market has closed for cleaning.

• See all of the updated exposure sites here

It comes less than a month after the market was forced to close due to another COVID-19 outbreak.

Meanwhile, the state’s health department said COVID-19 fragments had been detected in the Camberwell-Balwyn area of Melbourne’s eastern suburbs, and two catchments in the city’s west, including Tarneit, Truganina and Williams Landing.

-AAP