There are "very real concerns" for two Australians affected by the London attacks.

There are heightened fears for a young Brisbane woman who may be a fifth Australian injured in the weekend’s London terror attacks.

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

Her family had hoped Ms Zelenak may have been one of two unnamed Australians caught up in the dramas but Prime Minister Malcolm Turnbull on Monday afternoon said the government had been in touch with those families.

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.”

The frantic appeal came as the Queensland families of two other Australians – Brisbane’s Candice Hedge and Darwin electrician Andrew Morrison – breathed major sighs of relief after they were stabbed.

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 or arteries.

“She can’t think how she got so lucky because she thought she was going to die,” Ms Hedge’s grandfather Brian, who lives in Queensland’s Darling Downs, told AAP.

Mr Hedge spoke to his granddaughter, who has been living in the UK for just over a year, after ringing her in hospital on Monday morning.

“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,” he said.

Mr Morrison is on his way home after receiving stitches for a stab wound he received while leaving a bar after watching the Champions League soccer final.

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 he was thankful his son was heading home, and expect him back on Tuesday.

“It could have been worse, a lot worse,” he told ABC radio.

Seven people were killed and 48 wounded when three men launched the attack just after 10pm on Saturday local time.

Police shot the three terrorists dead within eight minutes of the violence erupting.

Mr Turnbull says the government has “very real concerns” for the other two unnamed Australians but wouldn’t divulge details of the pair.

“This is the work of cowardly, crazed criminals,” Mr Turnbull told reporters in Sydney.

Their evil message was a “blasphemous corruption” of Islam and millions of Muslims around the world, he said.