More than 200 Victorians have been told to get tested for Covid-19 after "strong and unexpected" fragments of the virus were detected in wastewater.
Some 246 people in Melbourne's western and northwestern suburbs have been contacted by authorities and told to get tested as a precaution.
"This additional action is being taken due to the strength of the wastewater detection and because a known positive Covid-19 case, from flight QF778, has been in Victoria in the past 14 days," the health department said on Thursday.
The 246 people who were contacted include four close contacts of the positive case and 242 recently returned red and orange zone travel permit holders.
They have all recently returned negative results, but the department wants them to get tested again "out of an abundance of caution".
"It is most likely that these new detections are due to a person or persons continuing to shed the virus after the infection period," it said.
"However, it could also be due to a person living in or travelling through the area in the early active infectious phase."
Covid-19 fragments detected in Victorian wastewater
Northwestern suburbs Glenroy, Hadfield, Oak Park and Pascoe Vale between April 20-27
Benalla between April 10-15 and 20-26 (repeat detections)
Western suburbs Altona, Altona North, Brooklyn, Newport, South Kingsville, Williamstown and Williamstown North between April 20-27
Northern suburbs Briar Hill, Bundoora, Diamond Creek, Greensborough, Lower Plenty, Macleod, Mill Park, Montmorency, Plenty, South Morang, St Helena, Viewbank, Watsonia, Watsonia North, Yallambie and Yarrambat between April 20-27
Outer eastern suburbs Chirnside Park, Coldstream, Kalorama, Lilydale, Montrose, Mooroolbark, Mount Dandenong, Mount Evelyn, Olinda, Yarra Glen and Yering between April 20-26
Western suburbs Albanvale, Burnside, Burnside Heights, Cairnlea, Caroline Springs, Deer Park, Delahey, Hillside (Melton), Keilor Downs, Kings Park, Plumpton, Ravenhall, Sydenham, Taylors Hill and Taylors Lakes between April 18-26
Northwestern suburbs Avondale Heights, Calder Park, Hillside (Melton), Kealba, Keilor, Keilor Downs, Keilor East, Keilor Lodge, Keilor North, Keilor Park, Sydenham and Taylors Lakes between April 18-26
Eastern suburbs Balwyn, Balwyn North, Blackburn, Blackburn North, Box Hill, Box Hill North, Bulleen, Doncaster, Doncaster East, Donvale, Mitcham, Mont Albert, Mont Albert North, Nunawading and Templestowe Lower between April 20-24
Outer northern suburbs Epping, South Morang or Wollert between April 17-22
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