In short:

The Borumba Pumped Hydro project will now cost $18.4 billion and will not be ready until 2033 at the earliest.

The 2,000-megawatt facility was a signature part of the former state government's plan to power two million homes.

A major renewable energy project in south-east Queensland is $4.2 billion over budget and three years behind schedule.

Work on the $14.2b Borumba Pumped Hydro project south-west of Noosa was due to begin next year and deliver electricity by 2030.

The 2,000-megawatt facility was a signature part of the former state government's renewable energy plan to power two million homes.

But a Queensland Hydro report, commissioned before the election has revealed the project will now cost $18.4b and will not be ready until 2033 at the earliest.

The "risk-adjusted" completion date is now July 2035.

"The re-forecast … accounts for significant external factors, including 'hyper-escalation' in construction costs and extended approval timelines," the report said.

Treasurer David Janetzki criticised the former government for "another blowout" to a major project. 

"In the report there, it actually said that the former government had a less than one per cent chance of actually delivering this project in the time frame," he told ABC Radio Brisbane.

"An independent consultant thinks it could even by north of $19 billion."

Mr Janetzki said his government remained committed to building the facility.

He said the report highlighted "other smaller, manageable options" but not specify what they were.

"I've got a meeting with Queensland Hydro tomorrow and we're going to start exploring those smaller options," he said.

The former government awarded $215 million worth of contracts in September, but refused to confirm the start date. 

The project has not yet been given federal environment department approval to start exploratory works.

How the scheme would work

A total of seven dams would be required for the project on the Mary River system, including an "upper reservoir" 1.5 times the size of the existing Borumba Dam.

The proposal has one main reservoir, a primary saddle dam and four secondary saddle dams straddling low points to contain water that could escape or overflow from the reservoir.

A series of tunnels to an underground powerhouse would pump water uphill to the new mountaintop reservoir and release it to drive turbines and generate electricity.

Scientist and national Landcare award-winner Steve Burgess has devoted decades to protecting and improving the Mary River catchment that flows out to the Great Barrier Reef and is the fourth-highest source of sediment on the reef.

He welcomed the release of Queensland Hydro's revised costings for Borumba and said the $4.2b budget blowout did not come as a surprise.

"It's good to see reports like this being published, because we've always wanted to see more transparency about the details behind the project," Mr Burgess said.

Call to make reports public

When the LNP government axed the Pioneer-Burdekin Hydro project, Deputy Premier Jarrod Bleijie immediately released the detailed analytical report (DAR) for the rejected proposal.

Queensland Treasury has provided the ABC with a copy of the executive summary of Borumba's cost and schedule re-forecast, but declined to provide further details for confidentiality reasons.

Repeated requests have been made for the DAR for Borumba Hydro to be released.

"If this is going to be a genuine process, all of the relevant reports should be made available to the public," Mr Burgess said.

He said the dam wall failure assessment for the Borumba Pumped Hydro proposal was completed in 2022 but had not been made public.

"You would think that's important information that people would need to have access to when they're looking at the consequences or impacts of this project on the wider landscape and community," Mr Burgess said.

He has called on the government to consider building a balancing storage dam on neighbouring Borumba creek and connecting it to the upper reservoir for the exclusive purpose of hydro pumping.

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The Mary River Catchment Coordinating Committee's alternative proposal would avoid destroying up to 1,481 hectares of threatened species habitat and 297 hectares of listed forest ecosystems, protected under the Commonwealth Environment Protection and Biodiversity Conservation Act.

Taking Borumba Dam out of the pumping process would prevent creating tides of between two to 10 metres a day, which would cause erosion and send sediment down the Mary river to the Great Barrier Reef.

"We hope in the redesign they are taking the environmental risks very seriously and not solely focusing on the economic risks," Mr Burgess said.