A rash of arson attacks in Melbourne's northwest is draining critical emergency service resources and could cost lives if it continues, authorities warn.
Four fires were lit in an industrial area at Sunshine on Wednesday night, across the valley from where a string of fires were deliberately lit at Keilor East the night before.
Residents at Keilor East were left panicked, distressed and in tears overnight on Tuesday as a blaze threatened several homes, including an aged-care facility.
In Wednesday's fires, grass and piles of rubbish on private property were torched as well as two cars parked nearby.
Metropolitan Fire and Emergency Services Board (MFB) deputy chief officer David Youssef said authorities were extremely concerned about the attacks, particularly ahead of dangerous weather conditions expected on Friday.
"These fires pose a huge risk to the Victorian community, both in terms of potential loss of life and also loss of property, and they take up what are critical emergency services resources," he told reporters in Melbourne on Thursday.
"It drains the Victorian fire service resources, it means trucks aren't available for other activities.
"It may well cost lives if it keeps going."
Mr Youssef said it was not known whether one person or several people were responsible for the arson, but he said the area had previously had problems with young people lighting fires.
Police are investigating reports of youths having been in the area when the fires were lit at Keilor East and say the same offenders may be responsible for the fires at Sunshine.
Assistant Police Commissioner Steve Fontana said fires caused by irresponsible behaviour were also a major concern.
"Particularly on days of severe weather and extreme heat, we've got people that might be using equipment, slashing grass, using angle grinders, (throwing) cigarette butts or driving vehicles in long grass, and it's causing these fires," he said.
"It's irresponsible and it's putting people at risk."
Mr Fontana urged members of the community to be on the lookout for suspicious activity in their areas and to contact police.