Victorian Premier Ben Carroll was among those who attended the vigil in Melbourne's Federation Square on Monday night.
Mr Carroll announced a $100,000 donation from the Victorian government to help with the search effort in Nepal.
"All of Victoria stands behind you, and every candle in Federation Square tonight is lit up for you, Mr Carroll told the crowd.
Foreign Minister Penny Wong told the ABC on Monday night that it remained difficult for the government to get updates on the 43 Australians who were missing, despite the work of the Nepali government.
She said this was due to difficult terrain and the extent of devastation caused by the floods.
"We do hold grave ... concerns for their safety, but we are determined to do everything we can to try and ascertain their whereabouts," Ms Wong said.
Australia has sent a 12-person response team and pledged millions in aid, including hygiene kits and solar lights to be delivered by the defence force later in the week.
Among the Australians still missing are four members of the Iyer family, Sreedharan, Lakshmi and their sons, Rudra and Rishabh.
Rishabh's partner, Sarada Vishwanathan, said the family were upset, disappointed and very scared.
"The main thing we want now is to try and get the Australian government to go over there, send their troops over to find our people," Ms Vishwanathan told Nine's Today program on Monday.
"It's been five days now, and we know that the critical window of survival is getting slimmer and slimmer as the days pass. And we just have no answers."
Ms Vishwanathan's partner and family were visiting the area for a religious pilgrimage to Mount Kailash in Nepal, but they had not been seen nor heard from since Wednesday morning.
Sydney IT worker Gopi Krishna Naidu Agastya, Royal Australian Air Force member Karan Bhardwaj, Sydney woman Sumi Adhikari and Sunshine Coast woman Renee Spry have also been confirmed among the Australians missing in the tragedy.
Organisations from the Emergency Action Alliance, which includes major Australian aid providers, are operating at a large scale for the first time, drawing on significant financial support from the government and public donations.
Save the Children Australia chief executive Mat Tinkler said the focus, for now, was on life-saving measures like search and rescue operations, along with water, shelter and food distribution.
Long-term efforts would focus on recovery, reuniting children with their families and rebuilding infrastructure.
"Having a joint platform allows us to maximise public goodwill and minimise duplication and overheads, so that gives the public great confidence that their money will flow through to who needs it most," Mr Tinkler told AAP.
"At the moment, we're in the acute life-saving phase where people are focused on search and rescue for the victims and things like water, shelter, blankets for those who have survived.
"These are things that people need in the first few weeks or so, when often they've fled the floods with nothing but the clothes on their back."
As many as 70 schools had been destroyed by the floods, he said, and his teams on the ground had already reunited 70 children with family members after being displaced by the disaster.
Assistant Foreign Affairs Minister Matt Thistlethwaite confirmed 43 Australians were missing following the disaster.
"Our embassy staff in Kathmandu are working around the clock and we've sent an additional 12 personnel to Kathmandu to assist," he told ABC Radio, expressing hope for their safe return.
Australia added $3 million to its aid package, mostly to supply hygiene kits and solar lights, with survivors urgently needing clean water and medical supplies.
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