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The world number one was sick in a 12-minute break while the Rod Laver Arena roof was being closed because of rain.
"I've got a really bad cold," the American said after her 6-3 7-6 (7-5) win over Russia's Maria Sharapova.
"I ended up throwing up. I think that helped when I got everything out of me and cleared my chest out because I couldn't breathe a little bit."
Williams fired down an ace when play resumed with the score at 30-30 in the sixth game and won six successive points to take a commanding 5-2 lead in the opening set.
The 19-time Grand Slam champion also won the first six points of the second set before Sharapova fought back and deservedly took the set to a tie-break.
But, after saving a championship point in the 10th game and repeating the feat in the tie-break, Sharapova eventually succumbed to Williams's serving power as an 18th ace won the match.
However, the 33-year-old said her illness would lead to muted celebrations as she wanted to "go to bed, curl up under the covers and go to sleep".
"I've got a bad cough," she said. "Usually you stay in bed, you don't play matches and practise every day."
Sharapova reflected on a 16th successive defeat by Williams by saying she was "happy" to be in the final because it meant she was "doing something well".
The 27-year-old, who has won five Grand Slam titles, including the 2008 Australian Open, said: "If I'm getting to the finals of Grand Slams and setting myself up to play a match against Serena, I'm happy to be in that position.
"Yes, I haven't won against her many times, but I'm not just going to go home without giving it another chance.
"That's just not who I am and not who I was raised to be. I'm a competitor.
"It's not easy to get to the final of a Grand Slam. That's a good achievement. It's a good start to the year. The year is very young at this point."
Serena Williams said vomiting during the final helped her win a sixth Australian Open title in Melbourne.