Two dead after one-day Sydney dance music festival.

Two people have died and another two are critically ill in hospital after multiple suspected overdoses at Sydney music festival Defqon.1.

A man, 23, and a woman, 21, collapsed at the festival in Castlereagh about 9pm on Saturday and died a short time later in nearby Nepean hospital, police said on Sunday.

A Jamisontown woman, 26, remains in a critical condition at Nepean hospital while an Artarmon man, 19, was flown to Westmead Hospital and is in intensive care.

Police said another 13 people went to hospital for drug-related issues while about 700 people sought assistance from medical staff at the festival.

Defqon.1 organisers had warned ticket holders the festival’s drug policy was zero tolerance.

“This means that all types of soft- and hard drugs are prohibited. If drugs are found, you will be handed over to the police,” a statement on the festival’s website said.

Police say 10 people were charged with drug supply offences, including two 17-year-old girls who allegedly carried 120 capsules “internally” into the venue.

In total, 69 people were found in possession of drugs at the festival held at Sydney International Regatta Centre.

Local detectives have formed a new police strikeforce, dubbed Highworth, to investigate the two deaths.