Earlier opinion polls have shown Gillard is on track to win the August 21 election, where she is seeking voter endorsement after she ousted Kevin Rudd, the former prime minister, in a dramatic party room challenge last June.
A Nielsen survey on Saturday found Gillard had consolidated her lead in the first week of the campaign, while the Reuters Poll Trend on Thursday found Gillard was set to win a larger majority on August 21.
Both Abbott and Gillard have promised to cut immigration numbers in a move that has angered Australian business.
Abbott vowed to curb the immigration intake from 300,000 people last year to at least 170,000 a year, and to ensure population growth slows to around 1.4 per cent from more than 2.0 per cent.
"I certainly have said we need a sustainable Australia, not a big Australia"
Julia Gillard, Australia's prime minister
"What we would do is manage the programme to bring it down to what we think are sustainable long-term numbers," Abbott told reporters on Sunday.
But Gillard hit back saying the immigration intake was already falling to around 230,000 people this year and would fall further to around 145,000 people by 2012.
"I certainly have said we need a sustainable Australia, not a big Australia," Gillard said.
"We've brought immigration numbers down as the economy obviously was dealing with the impact of the global financial crisis, down to 230,000."
Australia has been going through an immigration boom as it seeks skilled workers in a tight labour market, and Rudd had championed population growth from about 22 million to 36 million by 2050.
But the move upset many people in crowded outer suburbs around the biggest cities of Sydney, Melbourne and Brisbane, where government services are struggling to keep up with growth and where housing is becoming less affordable.
Around one in four Australians were born overseas, including both Gillard and Abbott, who were born in Britain but immigrated to Australia as children.
The Business Council of Australia, which represents the country's biggest industries, said the tough talk on cutting immigration was not in the national interest.
Katie Lahey, the council chief executive, said in a statement: "Growth will offset the effects of Australia's ageing population and ensure that governments have the revenue they need to pay for health care, education, infrastructure and environmental initiatives."