A young Auckland police officer is on trial accused of assaulting a woman with a Taser at near point-blank range after a high-speed chase led to a confrontation at the SkyCity Casino.

It was during the early hours of September 17, 2017, that Constable Sean Mathew Doak found himself in pursuit of a speeding Subaru car.

The chase began in Mt Eden and ended in the underground carpark of the downtown Auckland casino after police spiked the suspect vehicle's tyres.

It is what happened next which has led to the 25-year-old Doak sitting in the Auckland District Court today - not as a sworn member of the police but as a defendant in a criminal jury trial.

He was charged last year with assault with a weapon, a Taser, and of illegally presenting a restricted weapon.

Prosecutor Bruce Northwood described the Crown's version of events about what happened inside that carpark.

He said after pursuing police stopped the vehicle the driver of the car, a man, "bolted from the scene, leaving behind the young woman".

The young woman was Mary Jane Takerei.

Doak and his partner, a fellow constable, were the first on the scene, the court heard.

However, Doak quickly "took off, possibly looking for the driver", Northwood said.

His partner and other police officers who later arrived at the scene apprehended Takerei - dragging her by her feet from the car, Northwood said.

"This was all captured on high-quality CCTV," he said. "The woman was lying face down on the ground, one officer either side."

Northwood said, "fairly early on and without too much difficulty" Takerei was restrained and "ceased to be any kind of threat to the police officers present".

Doak then returned to the scene, the court heard.

He knelt down beside Takerei with his Taser drawn before threatening to use the restricted weapon near her head as he attempted to extract information from her, Northwood alleged.

The young officer was demanding to know where the fleeing driver had gone, Northwood said.

"Not surprisingly, the young woman didn't know where the young man had gone. She, however, was petrified.

"All she could do was hand over a name."

Northwood said the jury would hear evidence about the availability of Tasers for police officers, and how the deployment of the electric shock weapon is guided by written policy.

"To put it simply the use of a Taser is not a free-for-all," he said.

The wanted driver, the court heard, was later found by police coming out of one of the elevators in the casino as he attempted to slip away.

Doak and his partner, meanwhile, were tasked with transporting Takerei home.

But Northwood said, "once again [Doak] drew his Taser in the [patrol] car" and activated it.

When the constable did this he fired the arc option, which triggers the Taser to produce an electrical current between two prongs, Northwood said.

A recording from the weapon will show it was used in the arc mode, the prosecutor added.

"He let his professionalism slip badly that night," Northwood said. "[Takerei] was no threat to him, to others or even to herself. [Doak] used his taser in two clearly unacceptable ways."

However, Doak's counsel, Todd Simmonds, told the jury his client was simply "a young cop doing what cops are meant to do".

He contested Takerei and the Crown's version of events and said the young woman had been "vigorously resisting arrest on the ground".

"[Doak] kneels down and is demanding to know where is the male and where has he gone, who is he?

"Doing what police officers are expected to do," Simmonds said.

He added while the Taser was drawn and in Doak's hands there was "no threat".

Referring to the incident later in the patrol car, Simmonds said Doak accepts he pulled the trigger "for about a second".

But Simmonds argued there is no evidence to suggest Doak presented the Taser at Takerei in the back of the police car.

"It was a breach of police policy and he accepts that. And he will have to deal with that internally, that's not why you're here," Simmonds told the jury.

"He will have to answer to his bosses. You're not here to be caught up in police policy and what is a breach of that."

Superintendent Karyn Malthus, the Auckland City District commander, earlier told the Herald an internal police employment investigation will also be conducted pending the outcome of the court proceedings.

The Independent Police Conduct Authority is also carrying out its own investigation.

The trial, meanwhile, continues and is scheduled to conclude later this week.