An opinion poll published just a day earlier showed that the ruling Labor party and the Liberal-National opposition were evenly split after a hard-fought five-week campaign.
"I think it's too close to call at the moment. I think this is really, really tight," Chris Bowen, the Labor campaign spokesman, said.
"I think this will go right down to the wire and be the closest election in 50 years."
The Newspoll survey for The Australian newspaper suggested that around 44 per cent of voters put the opposition as their first choice against 38 per cent for Labor, but after other ballots were reallocated using the voter preference system they emerged almost neck-and-neck.
The poll of 1,600 voters was carried out on Tuesday and Wednesday, after a Newspoll at the weekend put Labor ahead by four percentage points.
A Galaxy poll for the Herald Sun newspaper on Thursday gave Labor a 52 per cent lead to the opposition coalition's 48 per cent.
Speaking to reporters in Sydney on Friday, Gillard acknowledged that there was a real chance that her party could lose its eight seat majority.
"What we know from the opinion polls is this, that there is a real risk that Mr Abbott could be prime minister on Sunday," she said.
Both sides are targeting a swathe of marginal seats in resource-rich Queensland, the home state of Paul Rudd, who was ousted by Gillard, and western Sydney, where rapid population growth has put pressure on services and raised concerns about immigration.
Al Jazeera's Dan Nolan, reporting from Melbourne, said that Gillard was likely to be hit by a protest vote against the ousting of Rudd.
"It could allow in the opposition leader Tony Abbott. Just a few months ago no one really thought he could be prime minister, but now it is there on a knife-edge," he said.
Abbott, who doubts the science behind climate change, has attacked Labor over immigration and has told voters that the ruling party is planning a carbon tax that will lead to higher power bills for everyone.
|Tony Abbott, left, has attacked Labor over immigration and a alleged carbon tax [AFP]
He is the Liberal party's third leader since it lost power, but the first to threaten the government in opinion polls and on Friday he made a final push for support.
"I have been on a blitz over the last 30 hours or so. I've done six TV interviews, 14 radio interviews and I've visited 10 electorates," he said.
"I am running for the biggest job in the country, and if you're running for a big job, you've got to make a big effort."
Some polls have suggested that the election may result in a hung parliament, but most analysts expect Gillard's centre-left Labor party will hang on to power for a second three-year term with a slim majority.
Labor gained power in 2007 with 83 seats after 11 years in opposition.