Both the Labor and Greens candidates for the seat - who will potentially benefit from the calls - disavowed any knowledge of the robocalls, which directly target Ms O'Dwyer over budget changes to superannuation. Kelly O'Dwyer, assistant treasurer and sitting member for Higgins, has been the target of the union robocalls. Credit:Ben Rushton "For the first time in memory the government is proposing to make retrospective changes to our superannuation, changes that will affect over 1 million Australians," states the message. "This is unprecedented and absolutely unfair. "Your local member, Kelly O'Dwyer, is assistant treasurer and minister responsible for superannuation," it states. "So this election you have a unique opportunity to send a message to the government that these changes, which will directly impact you, are just not on. On July 2nd, I urge you to put the Liberals last. After all, that's where they are putting you." ACTU President Ged Kearney said the union movement had a "long history of campaigning for decency in retirement".

"As we have done in the past we contracted the services of a reputable company that uses publicly and commercially available databases to source numbers for the robocalls." Greens candidate for Higgins Jason Ball with leader Richard Di Natale. Credit:Justin McManus The government's proposed super changes will raise about $6 billion and include a $1.6 million cap on tax-free super, a $500,000 limit on post-tax deposits, lowering the cap for concessional contributions to $25,000 and changes to the transition-to-retirement scheme that will see it attract a 15 per cent tax rate, prompting a blow back from the Coalition's base. Greens candidate for Higgins Jason Ball said he was aware of the extensive robocall campaign against Ms O'Dwyer but denied any involvement. Illustration: Ron Tandberg

"We definitely agree with the sentiment of the calls and it shows what a key fight Higgins is going to be in this election," said Mr Ball. " I can tell you we are as surprised as our volunteers who live in the seat, who received the robocalls on Monday night. We knew nothing about it." Mr Ball said the calls showed that Higgins was going to be a tougher battle than first thought. Carl Katter, Labor's Higgins candidate, says he had no knowledge of the robocalls. "They are worried, the pressure in on Kelly O'Dwyer is mounting and they are worried about that." Labor candidate Carl Katter - the brother of maverick independent MP Bob Katter - insisted the ALP was in a better position than the Greens in the seat, based on respective primary votes in the 2010 and 2013 elections.

"I'm hearing a lot of angst about super. I think there are only a few electorates where these changes are felt deeply, and I would say that Higgins is one of them," he said. "Two people called me about the robocalls. They said they didn't appreciate the robocall, but I don't know anything about it." Higgins has been held by the Liberal Party since 1949, and the seat has not gone to preferences in more than three decades. At the last election, Ms O'Dwyer won 54 per cent of the primary vote. But changes to boundaries have allowed the Greens to make inroads and Greens leader Richard Di Natale has singled it out as one of eight lower house seats the party hopes to claim in the next decade. Follow James Massola on Facebook