The private paramedic company contracted to provide first aid services to the Stereosonic music festival has banned its staff from talking to NSW Police after the death of Sydney pharmacist Sylvia Choi.

An internal email seen by Fairfax Media reveals that hours after the 25-year-old's suspected drug overdose on Saturday night, Event Medical Services gagged staff from making any comments about Stereosonic.

"If you are approached by the police (which has happened before) do not make any comment," the email told more than 55 staff members who worked at the festival in Sydney Olympic Park. More than 48,000 people attended the event.

Paramedics were also not told they were being paid $30 an hour until just before the festival kicked off. Student paramedics hired by the company for the festival were on even lower rates.

On Sunday NSW Police Assistant Commissioner Frank Mennilli told reporters there was no delay in getting medical attention to the affected revellers.

Founded by paramedic and registered nurse Mike Hammond in 2012, EMS advertises itself as Australia's "leading specialists" in rave events.

"Experience trance music like never before, but don't forget about the PARAMEDICS," its website says.

On Monday, Mr Hammond defended the company's staff email.

"They [NSW Police] come to me, I own the company, they don't talk to anybody at the end of the queue, they call the head of the company."

"I'm really pissed off that you got that internal document," he said before declining to comment about other concerns.

In August the NSW Parliament passed legislation requiring paramedics to hold a minimum qualification.

"While these regulations are in the process of being finalised it is expected that they will require a person to have either a university degree or a vocational diploma in paramedicine," a health spokesperson said.

Mr Hammond in a letter to NSW Health Minister Jillian Skinner  last month wrote he was worried he may not qualify as a paramedic under the new rules.
 
"I am 57, I have been a paramedic since 1988 and yet under the new legislation, I face having to stop immediately and going back to school for between two and four years to gain a qualification for skills I am already able to perform," he said.

Gary Wilson, a secretary of the Australian Paramedics Association, said the "vast majority" of private contractors operated responsibly.

"The concern for the public is always those rogue elements that operate outside the system for a profit," Mr Wilson said.

It is understood private contractors are often less expensive than hiring the NSW Ambulance Service to provide extra care. The NSW service was also present on Saturday.

On Monday, friends continued to pay tribute on social media to Ms Choi, of Oyster Bay in Sydney's south.

Bec Herbert, who attended Gymea Technology High School with Ms Choi, said "the Sylvia that I knew was a fun, smart, friendly and happy girl who definitely had a lot going for her."

"I certainly did not expect her life to end in such tragic circumstances."

EMS is a separate company to Event Medical Services Australia, which also provides paramedic care.

NSW Police are preparing a brief for the coroner.

With Louise Hall