A taekwondo instructor has been charged with three counts of murder over the killing of a boy and his parents in Sydney.

NSW Police said a 49-year-old man, who had been under police guard at Westmead Hospital, had been charged with three counts of murder on Wednesday night.

A bedside hearing is scheduled for Thursday.

The boy and his mother were allegedly murdered at the studio before the killer got in his victim’s car and proceeded to kill the boy’s unsuspecting father at the nearby family home.

The allegations were detailed earlier on Wednesday by homicide detectives who were poised to charge the taekwondo instructor over the “harrowing” murders of the Korean-Australian family on Monday night.

“This was a loving family,” Detective Superintendent Danny Doherty told reporters.

“It was out of the blue. It was not something that was forewarned or flagged.”

The instructor was placed under arrest in Westmead Hospital, where he presented late on Monday claiming stab and slash wounds on his body were from a supermarket car park incident.

NSW Police will allege he assaulted the woman, aged 41, and the child with “murderous intent” in the North Parramatta taekwondo studio on Monday night.

He then allegedly drove her white BMW to Baulkham Hills and fatally stabbed the 39-year-old father of the child, before taking the car to hospital.

He was placed under arrest once the bodies were discovered on Tuesday.

The child regularly attended the taekwondo studio for lessons, Doherty said.

“We’re still establishing what other connections or what other relationships may have been or may not have been,” he said, describing Tuesday’s discoveries as a “tragic turn of events”.

“Three people from one family, it’s devastating.

“It’s been a harrowing experience for the family and friends of the three victims, they’re still dealing with this so I’m not going to speculate on any motive at this stage.”

Doherty sought further assistance from the public and encouraged anyone in the vicinity of the taekwondo studio on Monday night who may have seen or heard something to contact investigators.

Anyone with footage of the BMW on Monday night is also asked to contact police.

-AAP