Turia Pitt received burns to 65% of her body and was told she would never compete again after the 2011 ultra-marathon in Western Australia.
After 200 operations, she completed the 226km (140 mile) Hawaii event on Sunday
She has been hailed on social media as an "amazing role model".
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Ms Pitt completed a 3.8km swim, 180km bike ride and 42km run in a time of 14:37:30.
The mining engineer, author and motivational speaker completed the event using custom-made gear and brake levers on her bike to accommodate the severe injuries to her hands.
She also wore special race clothing to deal with the extreme heat and humidity.
"Because of my burns, I can't regulate my own body temperature so I've had to make some adjustments, or use standard tri gear in different ways," she revealed last month.
"I'm going to need things like cooling sleeves and white suits so I don't overheat."
After her encounter with the bushfire Ms Pitt spent 864 days in hospital and underwent many operations to treat her injuries.
She made her comeback in the Ironman Australia triathlon in May.
"Competing in Ironman has ultimately showed me that I literally can do anything I put my mind to," she said on her blog.
Ms Pitt was widely praised on social media as an inspiration.
"Such a great example of what can be done through hard work belief in one's self and good a support system," one person wrote on her Facebook page.
"You continue to amaze and inspire," said another.

The "Freital Group", named after the suspects' hometown near Dresden, consists of seven men and a woman.
The German nationals are accused of targeting refugees and left-wing groups in 2015, causing two injuries.
The female suspect confessed to carrying out the attacks, while the men refused to speak in court.
The alleged members - aged between 19 and 39 - face life imprisonment if found guilty of attempted murder, and up to 10 years if the court rules they were part of a terrorist group.
Their names have not been publicly released.
"They wanted to create a climate of fear and repression," federal prosecutors said last year as they prepared the case.
"The aim of this group was to carry out bomb attacks on asylum-seeker shelters, as well as on the apartments, offices and vehicles of those with differing political views."
The group is accused of targeting refugee accommodation in Freital and a left-wing housing project in Dresden with explosive devices in 2015.
In one of the assaults, the suspects threw explosives through the windows of a home for refugees, the prosecutors say.
The suspects are also alleged to have targeted a left-wing councillor's car in Dresden.
The trial is being held amid tight security.
In 2016, there were about 3,500 attacks against refugees and asylum seekers across Germany, the AFP news agency reports citing data by the German interior ministry.
More than 560 people - including children - were injured during the assaults.
An Australian runner who suffered life-threatening burns when she was trapped by a bushfire during a race has completed the Hawaii Ironman, seen as the world's toughest triathlon.
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Eight suspected members of a far-right group in Germany have gone on trial in the eastern city of Dresden, charged with terrorism and attempted murder.