Zelenak's stepfather Mark Wallace this afternoon shared a video of his stepdaughter on Facebook, saying he "really miss (her) funny little laugh".
One of Zelenak's best friends Sam Worthington also posted a heartfelt tribute online.
"Our beautiful Sara Zelenak.
"The closest friend to Bea and I since we were in year 7.
"You bought so much joy into our lives, to me an Bea for so long and also to Mum and Dad, who took you under their wing like no other."
Worthington said she couldn't believe she and her friend won't "get to grow old together" and recalled one of their last conversations.
"Just Thursday I messaged you and you told me everything will always work out," Ms Worthington wrote.
"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."
Foreign Minister Julie Bishop has confirmed two Australians were among the victims of the London Bridge terror attack.
Last night the family of South Australian nurse Kirsty Boden confirmed she was the first Australian victim of the attack.
Her family said the 28-year-old died after running towards the carnage to help victims who had been mowed down by a van and others who had been stabbed.
In a statement, Bishop said the government was "deeply saddened" two Australians had been killed.
"We continue to work with the United Kingdom authorities who have asked we wait for official confirmation of the identities of the victims, and for the families to be officially notified, before we officially release their names," Ms Bishop said.
She said the government was keeping in close contact with the families of the victims.
Seven people have been confirmed dead, and dozens of others are injured in hospital, following the attack on Sunday morning (NZ time).
Before the second death was confirmed, Zelenak's mother Julie Wallace revealed to radio station 97.3FM how her daughter had narrowly avoided being caught up in both the Manchester and Westminster terror attacks that rocked the UK earlier this year.
"The one with the policeman being stabbed about three months ago [at Westminster], she was there the day before in the exact spot," Wallace said.
"The one with Ariana Grande's [concert], she was going to go to the concert. Everything she's just missed."
Zelenak's friend Ms Gonçalves lost her immediately after the attack. She told Fairfax that the high heels she was wearing may have slowed her down.
"I ran thinking she would be running with me but I looked back and she wasn't there," she said.
Gonçalves fled through Borough Market and when she saw a terrorist stab a man there, she tried to call Zelenak via web-based messaging service WhatsApp. She attempted to reach her three times, but there was no answer.
Zelenak still hadn't checked her Whatsapp since five minutes before the London Bridge terror attack began.
"I wish I could have grabbed her hand," Gonçalves told Fairfax, saying that she felt guilty about having lost her friend.
Zelenak's friends raised $15,000 via a fundraising page to help with the search and to fly her parents to London.
Speaking to media yesterday outside the family's home in Ormiston, east of Brisbane, Zelenak's aunt Tara said she was an "absolutely beautiful" girl.
"The family is trying to keep it together but bracing for the worst obviously," she said before Zelenak's death was confirmed.
"Sara is absolutely beautiful. She is the girl next door, she is a very special kindred spirit.
"She's one of those people that doesn't drink, doesn't do drugs, doesn't do anything wrong.
"She's amazing and she's 21 years of age."
Zelenak's mother Julie Wallace had planned to visit her daughter in France later this month.
"We were counting down the days to see her," Wallace told Brisbane radio station 97.3FM.
"The last time I spoke to her was our time Friday afternoon at 4.42 and 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.
"She's a very smart, sensible girl. She is healthy, fit, trim, taut, terrific - she doesn't do anything wrong in any way.
"She's very sensible, far more sensible than I was at her age."
Wallace said she had reacted with "absolute horror" when she was notified that her daughter was missing.
"My husband and my boys are insanely distraught. We are a very close family but I have to stay strong for them, because that's what I have to do," she told 97.3FM.
Earlier, Prime Minister Malcolm Turnbull has confirmed that Zelenak was among four Australians believed to have been caught up in the attack and said the government had "real concerns" for her safety.
South Australian nurse Kirsty Boden, 28, has been confirmed dead after being attacked by terrorists on London Bridge. The other two Australians, Candice Hedge and Andrew Morrison, are recovering from injuries sustained in Saturday night's horror.
Seven people were killed and dozens more were injured when three men in a van drove into pedestrians on London Bridge and then went on a frenzied stabbing rampage in nearby Borough Market.
- with AAP
