"I'm sad to have to report that there are at least 17 people that at this stage we cannot account for," Efe news reported citing Victoria state Premier Daniel Andrews as saying in a press briefing.
"Their whereabouts is unknown to us, plus there is one person confirmed as deceased," he added.
Another person has been reported missing in the neighbouring state of New South Wales (NSW), the worst affected by the fire, where 15 people have died in the last few months, including seven since Monday.
One other death has been recorded in South Australia.
Authorities have ordered the evacuation of many areas in southern NSW before Saturday, when the conditions were expected to get worse.
"The priority today is fighting fires and evacuating, getting people to safety," Prime Minister Scott Morrison said in a press conference on Thursday.
Firefighters urged tourists to leave from the southern coast of NSW before it was too late, warning of dangerous conditions on Saturday with temperatures above 40 degrees Celsius combined with strong winds.
Images posted on social media showed long queues of vehicles on highways in the region, with people taking advantage of some reopened roads, despite poor visibility in parts due to the dense smoke.
Some 50,000 homes were said to be without power on the south coast of NSW, where some 2,500 firefighters are currently battling over 100 raging blazes.
Authorities in the state, where around 3.6 million hectares of forests, or 70 per cent of the total area destroyed nationwide, have been burned, have also ordered the evacuation of the Kosciuszko National Park due to the proximity of a bushfire which has razed 130,000 hectares of land.
In Victoria, the military has been deployed for the air and sea evacuation of around 4,000 people trapped at a beach in Mallacoota, a popular tourist resort town.
Australian Defense Minister Linda Reynolds tweeted that navy personnel were providing support to communities isolated near Mallacoota and transporting firefighters and the wounded, along with the photo of a medical professional alongside a helicopter on the deck of the navy ship "Choules".
Victoria authorities said that they expected to carry out air evacuations - currently hindered by the smoke - by the evening and sea evacuations on Friday.
Mallacoota, a coastal town of around 1,000 inhabitants in the East Gippsland, is the area worst hit by fires in Victoria, with close to 100 houses being destroyed.
( With inputs from IANS )