A major Australian study has found some of the nation's biggest polluters are meeting their emissions obligations using carbon credits that have not actually resulted in emissions reductions.

Almost a third of projects under Australia's carbon credit scheme did little to nothing to reduce emissions despite costing taxpayers hundreds of millions of dollars, the researchers found.

Andrew Macintosh, one of the lead authors of the paper and an environment law and policy professor at the Australian National University first sounded the alarm two years ago, calling the carbon market "largely a sham".

His calls were rejected by a government-commissioned review, but Professor Macintosh said the new research shows further evidence that human-induced regeneration – a core part of the Australian Carbon Credit Unit (ACCU) scheme – hasn't worked.

"The data is telling us very clearly that the credits that are being issued are in no way shape or form being matched by the abatement that these projects are actually generating," He told the ABC.

Researchers monitored 182 Human Induced Regeneration (HIR) projects, which make up about 30 per cent of all ACCUs and have cost taxpayers nearly $300 million over their lifetime.

They found many of the projects to grow native forests were claiming to be regenerating them in uncleared desert and semi-desert areas.

According to the study, 80 per cent of those projects experienced either no change, or negative change to their tree cover between 2013 when first registered, and June 2022, despite receiving nearly 23 million credits.

Researchers say study calls into question emissions results of biggest emitters

In October 2023 the government banned the registration of new HIR projects, but existing projects are still operating and generating carbon credits.

The research group's claims would have significant implications for the government's safeguard mechanism, the emissions cap placed on heavy emitters that forces them to either lower their emissions or buy credits to abate them.

The safeguard mechanism applies to facilities that emit more than 100,000 tonnes of carbon dioxide emissions a year in the mining, oil and gas, manufacturing, transport and waste sectors.

Professor Macintosh said the ACCU scheme was "severely lacking in integrity" and jeopardised the integrity of the safeguard mechanism.

"Most of the [safeguard mechanism] entities that are covered by the [ACCU] scheme will meet their obligations by buying carbon credits and surrendering them, rather than reducing their own emissions," Professor Macintosh said.

HIR projects received 37 million credits to June 2023, nearing a third of the issuances under the scheme, and covering a total area of around 42 million hectares.

The researchers found an increase of just 1.8 per cent in tree cover across those projects, based on satellite data since the projects were registered.

Professor Macintosh said those projects also didn't show a marked difference in regeneration from other nearby areas outside the credit scheme.

"We also compared the trends of tree cover inside the projects that have been credited to what's happening in comparison areas," he said.

"The primary driver is not the controlling of grazing areas but something else, and that something else is very likely to be rainfall."

Recommendations for ACCU monitor progressing

In 2022 after Professor Macintosh blew the whistle on the carbon credit scheme, the government ordered a review led by the former national chief scientist Ian Chubb.

That review, released in January last year, dismissed claims the scheme lacked integrity.