Sticking to existing policies for cutting Australia's emissions would cost the federal budget nearly $24 billion to meet promised targets.
A Parliamentary Budget Office costing done for the Greens shows the government would need to pump $23.6 billion into the Emissions Reduction Fund by 2030 if it were the sole means of funding pollution cuts.
Greens climate spokesman Adam Bandt said this suggested there was a "gaping black hole" in the budget.
"If the government wants to retain the Emissions Reduction Fund as the main mechanism to cut pollution, it will cost taxpayers billions," he told AAP.
"To meet its financial forecasts in the budget, the government either needs to drop the Emissions Reduction Fund or find an unaccounted for $24 billion to fund extra spending."
The government has repeatedly said the fund is the central component of its policies to address climate change and meet its target to cut emissions 26-28 per cent on 2005 levels by 2030.
The fund started with $2.5 billion in 2014 and now has about $300 million left after five rounds of reverse auctions for projects.
The government didn't top it up in this year's budget, saying it will wait for recommendations from a review of climate policies due to report at the end of the year.
Mr Bandt accused the coalition of having an empty vessel when it came to climate policy.
"Instead of making big polluters pay for their pollution, like the carbon price did, (Prime Minister) Malcolm Turnbull shovels public money to the big polluters and the bill to taxpayers has just gone up."
Meanwhile, modelling from energy and carbon advisory firm RepuTex has found Australia could cut emissions by up to 40 per cent on 2005 levels at no net cost to business - a target in line with expert recommendations of action needed to limit warming to two degrees.
It found many activities to reduce emissions - like installing solar panels, improving fuel efficiency and farm forestry - would benefit investors because they would cut their energy costs.
Businesses and individuals could save $13 billion by 2030 while the total cost to do all emissions reduction actions that cost less than $20 per tonne of carbon dioxide was just $2 billion, RepuTex research head Bret Harper said.
The firm believes in the absence of political policy leadership, business should look at a price of about $20-$60 per tonne of abatement as a reference point and take into account the risks of transition to a low-carbon economy.
"Ultimately businesses need to understand the cost of where we are going, not just the cost of where we are today," Mr Harper said.
