The 39-year-old woman was stabbed in the neck, head and back in the car park outside Crunch Fitness at Alexandria where, investigators believe, her 40-year-old attacker lay in wait.
Emergency services were called to the premises about 12.30pm on Wednesday.
The woman was treated by paramedics before being taken to hospital in a serious but stable condition.
Detective Superintendent Rodney Hart said police were searching for a man known to the woman and 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," he told reporters.
"So this has been treated as domestic violence-related."
The man was known to police for domestic violence-related offences, Supt Hart said.
"We believe he used a knife, a kitchen-style knife, and we are concerned that he may still be armed."
"It's horrendous what has happened to this poor victim," he added.
"To the offender, 'hand yourself in because you're going to be arrested'."
A witness said the woman was helped upstairs back into the gym, where staff began first aid. (Sam Lock/AAP PHOTOS)
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.
Another witness saw the woman 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.
Police remained at the scene of the attack on Wednesday night. (Sam Lock/AAP PHOTOS)
Detectives were still on scene as night fell on Wednesday, collecting and bagging items from the car park as well as the woman's car.
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.
Police have described the suspect as 170 to 175 centimetres tall, of muscular to solid build with dark hair and possibly a dark beard.
"Finding him at the moment is proving to be tricky," a spokesman said.
The man fled the scene in a grey 2006 Ford Focus hatchback with registration AYQ14W.
Detectives were reviewing CCTV footage.
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