Moments before the start of the Melbourne Cup, the semi-retired woman discovered she had won $212,737, after being dealt a royal flush - a straight of cards from the same suit from 10 through to ace.
"Nobody was able to watch the cup as the true action was on the Caribbean stud poker table," gaming manager Marc Yeoman said.
Casino staff said the woman, who was the only player at the table, thought she had been dealt a flush - which would have given her $100.
Instead the woman was told she had been dealt a royal flush all from the same diamond suit, and had scooped the jackpot - the largest payout in the casino's history.
Despite odds of more than 650,000 to one for the hand, it was the second time the woman had won the jackpot.
In February 2003, she won $110,752 after being dealt a royal flush of the hearts suit.
Shaken after her big win, the woman immediately requested a telephone to call her husband.
Casino manager Nick Winder said the winnings were paid out with $1000 chips and "it is a big moment when we have those in front of you".
Leaving with a cheque, the woman plans to use some of the proceeds to take her family on an overseas holiday.
"She is absolutely ecstatic," he said.
