The 45-year-old handed himself in at Dee Why police station at 7.50am on Thursday, nearly 24 hours after the attack across the city in Alexandria.
Police said his 39-year-old victim was knifed in the neck, head and back in the carpark outside Crunch Fitness on Wednesday afternoon after he lay in wait for her.
The woman was treated by paramedics before being taken to hospital in a serious but stable condition.
Detective Superintendent Rodney Hart said officers were treating the incident as a targeted attack.Â
"We believe they had been in the past - earlier this year - been in a very short domestic relationship," he told reporters on Wednesday.
A kitchen-style knife was thought to be used in the attack, Det Supt Hart added.
"It's horrendous what has happened to this poor victim," he said.
Witnesses reported hearing the woman scream as she was walking towards her car.
A witness said the woman was helped upstairs back into the gym, where staff began first aid. (Sam Lock/AAP PHOTOS)
"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 carpark 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 said.
Police remained at the scene of the attack on Wednesday night. (Sam Lock/AAP PHOTOS)
One man leaving the gym told AAP he knew the woman and said she visited the gym most mornings.
The alleged attacker has been transferred to the Manly police station but is yet to be charged.
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