EVEN after being hit with 50,000 volts from a Taser, Damien Lynagh couldn't be stopped from allegedly assaulting a police constable as she tried to do her job.

The young NSW officer, identified only as Constable Hansen, suffered a suspected fractured cheek, severe swelling and a head wound that needed nine stitches when she and her partner responded to reports of a domestic dispute.

The bloody images are the other side of the Taser debate - as some say the devices can be deadly, frontline police point out the devices are sometimes still not enough to defuse a situation.

Constable Hansen and her partner Constable Halpin were called to a domestic argument at Eastwood, in Sydney's northwest, at 1.15am yesterday. Police said they tried to separate the couple who were fighting.

Lynagh, 47, allegedly became aggressive towards police, forcing the officers to use capsicum spray.

When that didn't subdue him, Constable Halpin deployed and discharged his Taser.

Lynagh was charged with two counts of assault occasioning actual bodily harm, assaulting police and resisting arrest. He was refused bail and ordered to appear at Ryde Local Court later this month.

Constable Hansen, who has been in the force for just over two years, was treated at Ryde Hospital.

Assistant Commissioner Alan Clarke said Tasers did not always stop alleged offenders.

"Tasers have been a very effective tool in reducing assaults on police and reducing injuries," he said.

"But for a variety of reasons there are still occasions when police are injured despite the deployment of a Taser."

Mr Clarke said officers are trained to use verbal communication before firing.

"Wherever possible police will talk to offenders first. But if the situation escalates the use of a Taser may well be needed," he said.

He added that in more than 70 per cent of occasions where Tasers are drawn they are not used.

NSW Police are currently testing a new double-shot Taser which shoots two electric charges without the need for it to be reloaded.

The new device has been developed with a "back-up shot" that can be fired if the first charge misses or catches on the target's clothing.

The current police model comes with a similar back-up cartridge but it must be manually reloaded.