The date means her administration will serve a full three-year term, although analysts said the early notice meant she had lost her ability to surprise opposition leader Tony Abbott with a snap poll.
Opinion polls put Mr Abbott's Liberal-National party ahead of Ms Gillard's Labour party and she would be swept from office, losing up to 18 seats, if an election were held now. The government could concede power if it loses one seat.
The election will decide whether Australia keeps its controversial carbon tax, and a 30% tax on coal and iron ore mining profits, which Mr Abbott promised to scrap it if he wins power.
But apart from these two policies, the government and opposition differ little on domestic issues, and both support greater involvement with China, the country's biggest trade partner, and close defence ties with the United States.
Mr Abbott said he was ready to fight the election, adding it would be decided on Ms Gillard's credibility.
"This election will be about trust," he said, hinting he will focus on a broken promise not to introduce a carbon tax and failure to deliver a budget surplus.
Mr Abbott has eroded government support through his constant negative attacks, but has yet to make any detailed policy announcements.
He will make his first major speech of 2013 today.
Ms Gillard governs with support from a group of independents and the Greens, who all support the September 14 election.
That means she has locked in majority support until the election, although a sudden by-election could still change the balance if a lawmaker dies.
Under Australian laws, governments serve for up to three years and the PM decides the election date.