Labor have seized on reports of an early cost blowout in the proposed rebuilding of Sydney’s two major stadiums, saying there seems to be “no limit” to the state government’s spending on the controversial project.

The New South Wales government has faced strident criticism over its plans to knock down and rebuild both the ANZ and Allianz stadiums.

More than 180,000 people have now signed an online petition opposing the move, describing it as a “staggering” waste of public money that only benefits big business.

The NSW government said late last year its rebuild project would cost about $2bn, including $705m for the work at Allianz and $1.25bn for ANZ.

But the Australian reported on Friday that those figures significantly underestimate the full cost. Infrastructure NSW was reported to have conducted its own analysis, which found the cost would be hundreds of millions of dollars higher than first thought.

Infrastructure NSW is responsible for developing businesses cases for the two rebuild projects. The body did not dispute the accuracy of the report when contacted by Guardian Australia on Friday.

A department spokeswoman told Guardian Australia: “Infrastructure NSW is coordinating the production of business cases for the Sydney Football Stadium and Stadium Australia redevelopments. Once completed, the business cases will be presented to the government.”

ANZ Stadium in Sydney. Photograph: Paul Miller/AAP

The state opposition leader, Luke Foley, said the stadium spend shows the NSW government had its priorities wrong.

“We now learn that it’s to cost more than the two-and-a-half billion dollars that the government estimated these two new stadiums would cost,” he said.

“It’s just the wrong priority when there’s record waiting lists for elective surgery and at our hospital emergency departments.”

The NSW sports minister, Stuart Ayres, has been approached for comment.

The rebuild of Allianz stadium has been the subject of significant lobbying by the Sydney Cricket and Sports Ground Trust. The trust includes broadcaster Alan Jones and the former News Corp chief executive John Hartigan.

Allianz requires upgrading to meet fire and safety standards.

The stadium is rarely full, yet the trust has lobbied to rebuild it with a slightly bigger capacity and to expand its facilities.