Briony Scott, the principal of Wenona girls school in North Sydney, said the death of Georgina Bartter, 19, was "terribly sad" and she would be dearly missed by students and teachers.

Ms Bartter was found unconscious at Harbourlife festival at Mrs Macquaries Chair on Saturday and died in hospital.

"We are deeply saddened by the death of this gorgeous, funny, bright young woman," Dr Scott said.

"She will be remembered for her scholarship and for her active and friendly approach to life. She had an infectious sense of humour and we will miss her enormously.

"We have had a beautiful service for Georgina today with our Senior College students, and our younger girls have met with staff and counsellors.

"I thank our community for their kind words and offers of help, which we have passed on to the Bartter family.

"Our hearts and our prayers go out to them at this time."

Witnesses saw Ms Bartter collapse on the middle of the dance floor and shake uncontrollably about 4.30pm

Paramedics found her unconscious and took her to St Vincent's Hospital at Darlinghurst where she died.

Irish tourist Owen Mullins saw Ms Bartter just before she lost consciousness.

"I walked past this girl and noticed that she was not looking well, I stopped and asked if she was OK. She didn't respond," he said.

"I sat next to her and watched for about 30 seconds to one minute and returned to tell her friend that she needed help.

"She agreed and asked for a paramedic so I ran and got one. I thought she would be saved. This is terrible terrible news; she was so young."

It is understood Ms Bartter had taken a combination of drugs, and police say they knew she had taken at least 1½ ecstasy pills

Her parents, Simon and Kirsty Bartter, spent Monday at their family home in Longueville on Sydney's north shore, where many close friends and family arrived to comfort them.

Dozens of teenagers went to the waterfront home with large posies of flowers and tupperware containers filled with food.

Many hugged and held hands as they saw the Bartter family for the first time since learning of their friend's death.

In a statement, the family said the teen was "a beautiful and vibrant young woman, who was much loved and will be sadly missed".

"She had allergies and it was extremely out of character," the family said in a statement to Channel Nine

Mr Bartter's family used to own Bartter-Steggles, one of the largest poultry firms in the country.

Ms Bartter finished her HSC at Wenona last year and had just returned from a European holiday with friends.

Superintendent Mark Walton, commander of the Sydney City Command, said officers were alerted to the unconscious woman while they raided Harbourlife, which led to the arrest of 78 people for drug offences.

"Her family are in an absolute state of shock, as are her associates and friends. It's an absolute tragedy that we lose someone in these circumstances."

A Sydney emergency department doctor said young people were hospitalised every weekend after taking ecstasy.

Festival organiser, Fuzzy Events, said its staff reacted quickly to the woman falling ill.

"A drug overdose or adverse reaction is suspected.

"Right now our thoughts are with her family – we can hardly imagine the pain and heartbreak they must be feeling, and they have our deepest sympathy."

Ms Bartter's death comes about a year after a 23-year-old man died from a drug overdose at hardcore musical festival Defqon.1.

James Munro died after he took a number of ecstasy pills before attending the festival at Sydney's International Regatta Centre at Penrith in September 2013.