The battleground was drawn in Brisbane today for this month’s Australian general election, with opposition leader Tony Abbott announcing that immigration would be a key plank in his campaign.

Abbott, leader of the centre-right Liberal Party, said people smugglers could be jailed for 10 years or more as he launched his campaign, ahead of what is expected to be a closely fought election on 21 August.

Julia Gillard, who became the country’s first female prime minister after Kevin Rudd was ousted in a Labor party coup in June, was backed by her predecessor as the pair also hit the hustings in Brisbane.

Abbott was behind in the polls a month ago, but a recent poll by Galaxy research found a 50/50 split. Personal attacks on Gillard, combined with damaging revelations that she questioned key Labor policies, have seriously harmed her chances of victory.

Illegal immigration is shaping up as a decisive issue in an election which a recent poll suggested could leave Australia with its first hung parliament since the second world war.

Opinion polls suggest voters in marginal seats are concerned that the government is not doing enough to turn back boatloads of asylum seekers.

Abbott said a Liberal government would seek new talks with the tiny Pacific nation of Nauru to reopen a processing centre for asylum seekers, mirroring the plan which helped the party win the 2001 and 2004 elections.

“We are determined to send a strong message to people smugglers that their cruel and callous trade in human cargo must stop,” the opposition leader told cheering Liberal-National Party coalition supporters, under the banner: “Stand up for Australia, stand up for real action.”

Gillard was swift to dismiss suggestions Labor was soft on people smuggling, however, pointing out that the government introduced new border protection laws this year, including a maximum penalty of 20 years imprisonment for the most serious people-smuggling offences and up to 10 years in jail for assisting smugglers. “We’ve got mandatory jail sentences in the current legislation. We have had a crackdown,” she told Australian television.

Abbott also pledged to kill off Labor’s proposed 30% mining tax on the first day of a conservative government, and promised a united team if he wins office – seeking to draw comparisons with the perceived dissension within Labour since Gillard replaced Rudd. “It’s time to end this soap opera and to give Australia back a grown-up government,” he said.