Image copyright Supplied Image caption Candice Hedge is injured and recovering in hospital, her mother says

Australia has "very real concerns" for two citizens believed to have been caught up in the London terror attack, PM Malcolm Turnbull has said.

Relatives had been contacted as authorities sought "confirmation of their circumstances", he said.

"At this stage we are not able to say anything more," Mr Turnbull said.

Two additional Australians were injured. One, a woman, was recovering in hospital, while the other was a man already on his way back to Australia.

"There are around 130,000 Australians living in the UK at any given time, and many Australian visitors, and there were many Australians in the vicinity of the attack," Mr Turnbull told reporters.

Attackers condemned

The prime minister vowed his nation would stand in solidarity with Britain, describing the attackers as "cowardly criminals".

"We defy them and we reject the poisonous ideology that they peddle," he said.

The Australian recovering in hospital was Queensland woman Candice Hedge, according to her mother, Kim Del Toro.

Ms Hedge, who is in her 30s, had been stabbed in the throat.

"She was hiding at the time, and the guy got her, and stabbed her in the neck somewhere," Ms Del Toro told the Australian Broadcasting Corp.

"She is ok. She has had surgery and she is quite OK."

Meanwhile, family and friends of a young woman from Brisbane have posted on social media that they have not been able to make contact with her.

They said Sara Zelenak was separated from her friends on London Bridge during the attack and hasn't been able to be reached since.

The 21-year old had been working in London as a nanny.