"Horror, complete horror." Australian feared killed had close calls in other terror attacks.

The mother of a 21-year-old Brisbane woman who has been missing since Saturday’s attacks in London has revealed her daughter had two other ‘close calls’ with terrorism in recent months.

Sara Zelenak was last seen on London Bridge on Saturday night, when she became separated from her friends. No one has seen or heard from the young au pair since a van ploughed into a crowd and men began stabbing innocents at Borough Market.

The attacks left seven people dead, with only four victims so far identified.

In an interview with radio station 97.3FM on Tuesday morning, Sara's mother, Julie Wallace, described the moment she realised her daughter was missing.

The family Sara was boarding with in London called Julie on Sunday night to break the horrible news: Sara had never come home from her night out.

"Horror, complete horror," Julie said, before revealing that Sara had been in two other similar situations before.

"The one with the policeman being stabbed about three months ago [at Westminster], she was there the day before in the exact spot," Julie said.

"The one with Ariana Grande's [concert], she was going to go to the concert. Everything she's just missed."

A friend in London, Pri Goncalves, told Fairfax Media she is bracing for the worst, fearing Sara's high heels may have slowed her down and caused their separation during the chaos.

"I ran thinking she would be running with me but I looked back and she wasn't there," Pri said. "I wish I could have grabbed her hand."

She added, "I wasn't able to help her and it's killing me."

Sara's mum Julie is now on her way to London to search for her daughter, after she was originally told by police to wait 24 hours before jumping on a plane, despite her motherly instincts to travel overseas straight away.

"The police in London just said give it 24 hours because...the Australian [consulate] are closed over the weekend, so they don't do anything," she said.

"London actually didn't start working on finding Sara until 7am Monday morning.

"If I have information or not, I have to go over and do something, I have to go over there.

"My boys are insanely distraught. We're a very close family, but I have to stay strong for them because that's what I've got to do."

Julie said she last spoke to her daughter at 4:42pm on Friday afternoon, and the pair spoke up their upcoming trip to Paris.

"She said, 'Oh mum! It's only 28 days until you and I and dad can have baguettes and cheese and croissants in Paris. I'm so excited to go to Paris!' because she hasn't been," Julie said.

"She's a very smart, sensible girl, she is healthy, fit, trim, taut, terrific - she doesn't do anything wrong in any way."

A GoFundMe campaign has raised more than $15,000 to support Sara's family in their search.

If any Australians are concerned about family and friends and unable to contact them they should call 1300 555 135 or +61 2 6261 3305 if overseas.