A 19-year-old woman has died after taking a unidentified substance at the FOMO music festival in western Sydney.

Key points: Paramedics say the woman attended the festival's medical centre after having a reaction to drugs

Paramedics say the woman attended the festival's medical centre after having a reaction to drugs She is the fifth person in NSW to die at a music festival so far this season

She is the fifth person in NSW to die at a music festival so far this season Police searched 146 people at the festival, 54 were found with drugs

Alex Ross-King, from the Central Coast, was attending the festival at Parramatta Park when she collapsed about 6:00pm yesterday.

A NSW Ambulance spokesman said the teenager was presented to the medical centre at the festival site, appearing to have a reaction to drugs.

Paramedics treated her at the scene and in the ambulance on the way to Westmead Hospital where she later died.

Police believed the woman took an "unidentified substance" but were still confirming the circumstances behind her death.

Ms Ross-King is the fifth person to die at a music festival in NSW over 2018/2019 festival season. There has also been one fatality in Victoria.

The police are preparing a report for the coroner.

Friends described Alex Ross-King, 19, as a caring and bubbly person. ( Supplied: Facebook )

Friends described Ms Ross-King as a tenacious, bubbly and caring young woman.

"You were always full of life and a genuine person with such a big heart and quite easily one of the funniest people I've ever met," one wrote on Facebook.

Another described her as "the definition of a kindred spirit".

An estimated 11,387 attended FOMO — headlined by US singer Nicki Minaj — with officers searching 146 people and finding 54 of them with drugs.

Police said 36 people were arrested at the event, including three people charged for drug supply, while 27 were issued court notices for possessing drugs.

Premier still against pill testing

The woman's death comes as debate in New South Wales rages over the benefits of pill-testing at festivals.

The State Government has continually argued there is no evidence the harm-minimisation strategy would save lives.

In response to the latest death, Premier Gladys Berejiklian said it "tore her to shreds" every time a young person lost their lives to drugs.

"As the leader of this state, as the leader of the Government, my job is to keep the community safe at all times and if there's more we need to do we will," she said.

"But I also want to make sure we look at every opportunity to reduce those deaths. I worry that something like pill testing could have the opposite effect."

Asked about her preferred alternative to pill testing, Ms Berejiklian suggested there was opportunity to better educate young people about drugs and make sure they weren't embarrassed about seeking help if they got into trouble.

Last week NSW Labor leader Michael Daley said it would explore the possibility of pill testing if it won the March state election.

Festival had a 'support plan'

NSW Health said 10 other people presented to the hospital requiring medical attention after attending the festival and three of those remained there on Sunday afternoon in a stable condition.

The 19-year-old's death is the fifth fatality at a music festival in NSW this season. ( Supplied )

The department said a "comprehensive medical support plan" was put in place before the event, with the support of NSW Ambulance and Western Sydney Local Health District (WSLHD).

"WSLHD worked closely with the FOMO festival organisers to ensure frequent and extensive harm reduction and drug safety messages were issued to attendees prior to and during the event," the department said in a statement.

It warned festival-goers who attend upcoming events to seek help immediately if they feel unwell after taking a party drug.

MDMA had been implicated in several recent festival deaths and the department said its "lethal toxicity" was well known.

The organisers behind FOMO festival said the woman was with her family when she died.

US singer Nicki Minaj on stage in Parramatta last night ( Supplied )

In a statement, they said the safety and wellbeing of patrons was "at the forefront of every planning decision" and anti-drug messaging began weeks before the event.

"FOMO festival has always been clear that we do not condone the sale, supply or consumption of illicit substances," the organisers said.

Police reviewing festivals 'daily'

Assistant Commissioner Mark Jones said police were reviewing music festivals "daily". ( ABC News )

NSW Police said it was early in the investigation but they were pursuing a number of avenues to track down what the woman took.

A post-mortem examination will be conducted early next week and it is hoped toxicology results will shed light on the chemicals responsible for the death.

Assistant Commissioner Mark Jones said police were doing everything they could to try to make music festivals safe.

"We are reviewing these music festivals literally on a daily basis," he said.

"We're doing a large number of searches at events, we're trying to detect prohibited drugs.

"We're making it really clear to organisers they have to put in good practices to try to prevent both the use and the bringing in of drugs to these festivals."

Last year, the NSW Government introduced a new offence to target those who supply substances that kill people, which includes a jail sentence of up to 20 years.