Australian authorities are working with British police to establish the whereabouts of Brisbane woman Sara Zelenak and South Australian Kirsty Boden who are still missing after the London terrorist attack.
Prime Minister Malcolm Turnbull said there were real concerns about the two Australians after the weekend attack in which seven people were killed and 48 injured.
"We are working diligently in London with the police and other authorities there and, of course, staying in very close contact with their families as we seek to confirm the circumstances relating to those two Australians," he told reporters in Canberra on Tuesday.
Fears are growing for Ms Zelenak, who became separated from friends on London Bridge when a van driven by the terrorists mowed down pedestrians.
A Facebook post by a family friend shared hundreds of times on Monday appealed for information about the 21-year-old, saying she usually rang her mother daily.
Ms Zelenak's stepfather Mark Wallace said UK authorities had been unable to shed any light on where she was following the attack on Saturday night (Sunday AEST).
"I feel terrible, I can't think," News Corp reporting him saying.
"I've contacted every hospital in London but they can't give out patient details or even tell us if she has been admitted."
Mr Wallace said Sara's mother, Julie Wallace, was flying to London to try to find her daughter.
Ms Zelenak moved to London in March to work as an au pair.
Mr Wallace said she had been planning to babysit on the night of the attacks, but at the last minute the children's grandmother stepped in so she went out with friends.
Ms Boden, a health professional, is reportedly the other woman feared killed in the attack.
The 28-year-old, originally from Loxton in South Australia's Riverland region, had lived in London for some time, according to the Nine Network.
A Metropolitan Police spokesman in London told AAP that police had not yet released the names of the dead or injured.
"We are trying to locate people as best we can."
The spokesman said a casualty bureau had been set up to help people trying to find family members who may have been caught up in the attacks.
The frantic appeal for Ms Zelenak came as the families of two other Australians - Brisbane's Candice Hedge and Darwin electrician Andrew Morrison - took comfort in news they were okay.
Ms Hedge, 34, working as a waitress in the Borough Markets area, underwent emergency surgery but is expected to make a full recovery after her attackers missed her windpipe and arteries.
"She can't think how she got so lucky because she thought she was going to die," her grandfather Brian, who lives in Queensland's Darling Downs, told AAP after speaking to her by phone on Monday morning.
"She said 'grandad you know I'm a Hedge and I'm a fighter, I'll get over this."
Mr Morrison is on his way home and due to land on Tuesday after getting stitches for a stab wound received after leaving a bar.
He said he believed a brawl was breaking out when "all of a sudden a guy comes up with a knife ... stabs me there (motioning to his neck) I push him off and blood is going everywhere".
Mr Morrison's Gold Coast-based father Dave has told reporters "it could have been worse, a lot worse".
