A woman who allegedly attacked three people with an axe in Sydney told a friend an hour earlier that "one day I am going to kill a lot of people", a court has heard.

Evie Amati, 26, is on trial in the NSW District Court over the alleged attack at a 7-11 store at Enmore in inner Sydney in January 2017.

It is alleged the transgender woman strolled into the convenience store at 2:19am with an axe and used it to hit a man in the face and a woman in the back of the head.

The Crown said she then swung the axe at another man after leaving the store.

All three people survived.

The court was told Ms Amati had a history of mental illness.

In opening his case, crown prosecutor Daniel McMahon told the jury Ms Amati was listening to a song called Flatline by the band Periphery with "some pretty dark themes" before she went into the store.

He told the jury she did a lap of the 7-11 then spoke to customer Ben Rimmer who was waiting to be served.

Mr McMahon said as Mr Rimmer walked towards the counter Ms Amati, "swings [the axe] with force from her right to her left at Mr Rimmer's head".

"The blade of the axe impacts with the left side of his face ... penetrates through his skin and very soft tissue into bone," he added.

Mr McMahon told the jury the security camera vision showed Ms Amati then striking another customer, Sharon Hacker.

"There was a forceful blow to the back of Ms Hacker's head in the vicinity of the base of her skull," he said.

'I am going to kill a lot of people'

Mr McMahon told the court the prosecution would allege that when Ms Amati left the store she swung the axe at another man, Shane Redwood, who was able to run away after blocking the axe with his backpack.

He said he expected Ms Amati's lawyer, Charles Waterstreet, to argue she was suffering from a mental illness and had a history of gender dysphoria and depression.

"The accused will need to satisfy you that she was suffering from the disease of the mind at the time of the incident," he told the jury.

Mr McMahon said that before the incident the accused, "had fantasies about killing herself and others" and had spoken about twisting people's necks on the bus in 2016.

In the hours leading up to the incident, the court heard Ms Amati had been drinking and taking drugs with friends, and had learned someone she wanted to date was in a relationship with someone else.

One hour before she went to the 7-11, Mr McMahon said she messaged a friend to say, "most people deserve to die, I hate people and one day I am going to kill a lot of people".

Then about half an hour before she allegedly updated her Facebook status to say, "Humans are only able to destroy and hate so that is what I shall do".

Ms Amati broke down in tears several times as she listened to proceedings from the dock.

'She was out of her mind'

In his opening address, Mr Waterstreet told the jury his client was not guilty by reason of mental illness.

He said at the time of the attack she was in a state of psychosis caused by her mental illness and a "toxic mixture" of gender transition hormone medication, cannabis, amphetamines and alcohol.

"The CCTV captures the body of Evie Amati, it captures it from the time she entered the 7-Eleven to the time she left, so the question you have to decide is where was her mind?" Mr Waterstreet told the jury.

"Her mind had deteriorated to such a state that in the early hours of the evening in the 7th of January she was out of her mind."

He told the court his client was an intelligent woman who excelled in everything she did academically and had worked full-time for a union for seven years, but he added she had struggled mentally with her gender transition and was "suffering from a disease of the mind".

"This was a woman who had never done anything violent … it was a total out-of-character experience," Mr Waterstreet said.

The front row of the public gallery is full of people who have come to the trial to support Ms Amati.

The trial is expected to run for three weeks.