There are “very real” concerns for two more Australians caught up in the London terror attack, Prime Minister Malcolm Turnbull has confirmed, as fears grow for a Brisbane woman still missing after Saturday’s incident.

Family and friends of Sara Zelenak have taken to social media to appeal for information about the 21-year-old who hasn’t been heard from since witnessing the London Bridge attack.

A Facebook post written by a friend, and shared numerous times, says Ms Zelenak was last seen on London Bridge and witnessed the attacks before getting separated from her friends.

“Her phone has rang with no reply and now the battery must have gone,” read the original post, which has since been deleted.

“She is Australian aged 21 with long blonde hair. She calls her mum daily. It’s been over 24 hours with no news from the consulate. We was hoping the 3rd Australian reported would be her, but it’s not.

“She is based in London. Please share with as many people as possible especially if you have friends down south. Thank you for your help.”

Speaking to media in Sydney on Monday, Mr Turnbull said four Australians were now confirmed to have been involved in the attack.

In addition to Brisbane woman Candice Hedge, 34, and Darwin man Andrew Morrison, two other unnamed Australians were seriously injured in the attack, he confirmed. Neither of them is Ms Zelenak.

“There are two other Australians, so that brings it to a total of four, about whom we have very real concerns, but at this stage we’re not able to say anything more,” Mr Turnbull said on Monday.

The ABC has named a third Australian victim as Daniel O’Neill.

Seven people are confirmed to have died and at least 48 injured in the attack, with the attackers shot and killed by police within eight minutes of the first emergency call in what police said comprised “in the region of 50 rounds” of ammunition.

A policeman stands over one of the slain London Bridge attackers. Photo: AFP/via AAP

Assistant police commissioner Mark Rowley said 36 people were in hospital with a “range of injuries” with 21 in a critical condition.

Twelve people were arrested in east London on Sunday (UK time) in connection with the attack, with people releasing one man without charge.

British police said on Monday they will release the names of the attackers as soon as operationally possible.

Meanwhile, the UK’s Telegraph newspaper reports that one of the three attackers was reported to the anti-terror police at least twice.

The newspaper cited a former friend of the attacker who claimed he had been radicalised while watching YouTube videos and said that he had consequently contacted authorities ­after becoming concerned about the man’s views.

A neighbour also told the paper she had contacted local police after the man had tried to convert her children to Islam.

Australian woman thought she would die

Ms Hedge, 34, was working as a waitress in the Borough Markets area when three terrorists struck on Saturday night (UK time) and slashed Ms Hedge across the throat.

Expected to spend a week in hospital, Ms Hedge told her grandfather, Brian Hedge, she thought she was going to die.

“She told me she’s going well. She said ‘Grandad, you know I’m a Hedge and I’m a fighter, I’ll get over this,” Mr Hedge told AAP.

“She can’t think how she got so lucky because she thought she was going to die.”

‘Someone help me, I’ve just been stabbed’

Video footage of Mr Morrison, an electrician from Darwin, has gone viral after he was filmed shortly after the attack, describing how he tried to dodge the knife of one of the attackers before being sliced in the side of the neck.

“I walked outside, walked across the road and a fight breaks out. I said ‘Sh*t, what’s going on here?'” Mr Morrison said.

“I start to walk towards it, all of a sudden a guy comes up with a knife, and I … duck it, and he stabs me there.

“I just push him off, blood’s going everywhere. I walk into a pub, and I’m like ‘someone help me, I’ve just been stabbed’.”

WARNING: Mr Morrison’s account contains strong language

Canadian woman first named victim

Canadian woman Christine Archibald, 30, died in her fiancé’s arms after she was hit by the van driven by terrorists, his family has confirmed.

Cassie Ferguson Rowe, the sister of Ms Archibald’s fiancé, Tyler Ferguson, posted on Facebook confirming the couple was on the bridge when the van ploughed into the crowd.

Christine Archibald, 30, died in the arms of her fiancé, Tyler Ferguson (right). Photo: Facebook

Ms Ferguson Rowe says her brother held Ms Archibald, from British Columbia, as she died in his arms.

The Archibald family also released a statement confirming their daughter had been working in a homeless shelter until she moved to Europe to be with Mr Ferguson, and that she would have had no understanding of the evil that claimed her life.

– with agencies