A 39-year-old woman was stabbed at a Sydney gym this week. Her alleged attacker had breached an AVO.

A man has been charged over a suspected domestic violence-related stabbing attack outside a Sydney gym this week.

The 39-year-old woman was stabbed in the neck, head and back in the car park outside Crunch Fitness at Alexandria on Wednesday. Emergency services were called to the premises at about 12:30pm.

The woman was treated by paramedics before being taken to hospital in a serious but stable condition.

NSW Police issued a statement on Thursday morning saying a 45-year-old man had been arrested at about 7:50am, after presenting himself to Dee Why Police Station. 

He was taken to Manly Police Station where an arrest warrant was executed and was charged with causing wounding or grievous bodily harm to a person with the intent to murder, as well as contravening an AVO.

He was refused bail and will appear before Manly Local Court on Friday, May 10.

Detective Superintendent Rodney Hart said detectives were treating the incident as a targeted domestic violence attack.

"We believe they had been in the past — earlier this year — been in a very short domestic relationship," Detective Superintendent Hart told reporters.

"So this has been treated as domestic violence-related."

The man was known to police for domestic violence-related offences, Superintendent Hart said.

"We believe he used a knife, a kitchen-style knife... It's horrendous what has happened to this poor victim," he added.

Witnesses reported hearing the woman scream as she was walking towards her car.

"When I was about to open the door, up to the gym, I heard a scream... it was not a normal scream," one woman told reporters.

"Because of the distance, I wasn't sure what he was trying to do to her... it looked like he was attacking her or he was trying to rob her."

The witness believed the man was hiding in the car park, as she did not see him when she arrived. A second man stepped in to stop the attack.

According to another witness, the woman was helped upstairs into the gym, where staff began first aid.

"She ran all the way upstairs, she was bleeding from the neck, from her ears," he told reporters.

One man leaving the gym told AAP he knew the woman and said she visited the gym most mornings. 

He believed she had only been dating the alleged offender for a matter of weeks. 

If this has raised any issues for you, or if you just feel like you need to speak to someone, please call 1800 RESPECT (1800 737 732) – the national sexual assault, domestic and family violence counselling service.

Feature image: AAP.