The 26-year-old Montgomerie, a mother of two, was thrown from her Gary Clarke trained horse Riahgrand 100 metres from the finish line in the sixth race of the meeting.
She was treated on the course before being taken to hospital, where she fell into a coma and died around an hour after the fall.
There were three races left, which were cancelled as soon as the gravity of Montgomerie's condition became apparent.
"As far as we are concerned, Simone was our family and we are all devastated by this accident," said Darwin Turf Club chairman Brett Dixon.
"There will be a full investigation which has already been initiated by the stewards," the cluib added in a statement.
"Simone was a champion jockey at the top of her game and a pioneer of women in racing who will be sorely missed," Northern Territory chief minister Adam Giles said.
Montgomerie's father Peter is a trainer, with his On A Jeune the runner-up at the 2005 Melbourne Cup.
Champion jockey and former Melbourne Cup winner Kerrin McEvoy was devastated by the news.
""I'm shattered by the news," McEvoy said.
"I know Simone really well, we actually did pony club together. My condolences go to the Montgomerie family.
"She's a beautiful girl, it's just a tragedy."