The 21-year-old was starting the trip of a lifetime, planning to work as an au pair for five months as she travelled around Europe, including a few days to reunite with her family in Paris.

Her mother and brother cried on the way back from the airport, but found solace in the fact they would see her again soon - and that Sara had a large teddy bear with her, a gift from her brother on her 21st birthday.

He had told her to hug the bear if she missed him.

Sara, a loyal friend who loved to make people laugh, quickly made friends in London including a fellow au pair she met on a train, as everyone back home kept up with her adventures.

On June 3, her host family gave her the night off, and she headed towards London Bridge. About 10pm, she left a restaurant with one of her friends and was walking across the bridge when a terrorist attack unfolded in front of them. Her friend managed to escape, but the attack claimed Sara's life.

Like others in London, her life had been touched by two previous terrorist incidents.

In frequent phone chats with her mum, Julie Wallace, she said she had been sightseeing in Westminster the day before an attack unfolded there in late March. She also bought tickets to Ariana Grande's ill-fated concert in Manchester, but didn't end up going.

Sara was due to meet her family in Paris on June 30 to spend five days together, and spoke about her excitement in their last conversation the day before she died.

She couldn't wait to eat baguettes, cheese and croissants in the city of lights on her first visit there.

Instead, Mrs Wallace set off three weeks ahead of schedule to find her missing daughter in London. She was at an airport stopover when she was told her daughter had died.

"My beautiful daughter Sara Zelenak has been confirmed dead," she wrote on Facebook, alongside photos of Sara smiling at the airport before her flight in March.

"Sara was so excited about her European & UK adventure."

After meeting up with her family, Sara was due to go on a Contiki trip around Europe, before coming home on August 8.

"Unfortunately [this was] cut short by a senseless act of terrorism," Mrs Wallace said. "It makes no sense."

Hundreds of people have contributed to a crowdfunding page initially set up to fund the search for Sara, which will now cover the expenses of bringing her home.

Tributes have flowed on social media, with friends, family and former schoolmates remembering her love for R&B music and KFC, her quirky sense of humour and her love for travel.

One of Sara's close friends Elle Bishop said they became "instant friends" when they met, describing Sara as "such a character, with the funniest sudden bursts of laughter", who always had time to cheer up her friends with ridiculous stories.

"[She had] a great love for adventure and travel, seeing the world and living her life to the fullest, as all young people should be able to," Ms Bishop wrote on Facebook. "Always encouraged by those around her in moments of self doubt to chase the opportunities given to her, and oh boy she did, which is now how she will forever be remembered.

"I am, we are, so blessed to have you in our lives. I could think of no better Angel to watch over us all.

"Our forever loved, forever beautiful, forever young, Sara. You will be forever missed."

Another friend, Sam Hetherington, said Sara "brought so much joy into our lives".

"I can't believe we don't get to grow old together," she said. "Just Thursday I messaged you and you told me everything will always work out. You would be so overwhelmed by the love from everyone and probably be very salty from the dorky pictures we have together that are all over the world hahaha.

"Rest easy wherever you are, I hope there is lots of KFC and hot boys with you. Forever love."

Sharley Yak, a friend from Brisbane, wrote: "I'm not one to usually write these type of messages but when you lose someone as special as Sara Zelenak in your life, you need to find another way to express the pain inside."

He said he would "deeply cherish" the time he got to spend with Sara over several years.

Sara's former school, Moreton Bay College in Brisbane, wrote in an announcement that their "hearts are broken by the loss of Sara".

"Sara was a positive, popular student who always had a smile on her face. She embodied the values of Moreton Bay College and was adored by her peers and staff. This is how she will be forever remembered," they said.

The school will hold a memorial in their former student's memory, and said they will set up a tribute to her in the quadrangle.

Speaking in Sydney on Wednesday, Prince Harry paid tribute to Ms Zelenak and Kirsty Boland, a South Australian nurse who was killed running to help victims of the attack.

"I would like to start by sending my thoughts to those affected by Saturday's attack in London Bridge," Harry said.

"Australians form an important and vibrant part of the fabric of life in London and we are reminded of that in good times and bad.

"Our hearts go out to the victims, their friends and families."