Another detailed study has concluded that the Coalition's climate policy will cost vastly more than has been budgeted or else will dramatically fail to meet Australia's emissions reduction targets.

Modelling by Reputex climate analytics, commissioned by the environment group WWF-Australia, found that the money set aside by the Coalition to buy abatement was likely to fall short by $5.9bn a year between 2015 and 2020, or between $20bn and $35bn in total.

The Coalition insists it will provide no more than the money it has allocated to its emissions reduction fund – $2.5bn over the next four years – and, according to figures in the original 2009 policy document, almost $5bn by 2020.

According to Reputex, if it sticks with this funding allocation Australia's emissions will rise by 16% by 2020 compared with 2000 levels, missing by a long way Australia's international commitment to reduce emissions by 5% over that timeframe.

Both Labor and the Coalition have committed to the 5% reduction as Australia's minimum effort and have promised to reduce Australia's emissions by up to 25% if other countries also take strong action.

But Reputex found that a 25% cut could not be delivered by Direct Action under any realistic pricing scenario.

The fact that a direct action scheme cannot be "scaled up" was pointed out by former Liberal leader Malcolm Turnbull when he explained in 2011 that continuing to use a big government taxpayer-funded scheme to reduce emissions in the long term would "become a very expensive charge on the budget in the years ahead".

The Reputex analysis is more damning of the Coalition's plan that recent modelling by Sinclair Knight Merz/MMA and Monash University's Centre of Policy Studies, commissioned by the Climate Institute.

That modelling, which used assumptions more generous to the Coalition, found the Coalition would have to find at least another $4bn for its climate policy or else break its pledge to cut emissions by 5% by 2020 and instead allow them to increase by 9%.

Reputex also modelled the long-term costs of Labor's emissions trading scheme, finding it would meet the 2020 target with an average carbon price of $22 a tonne – slightly lower than the current fixed price. Labor has said it would bring forward to next June the date upon which the carbon price is set by the market, at which point it is likely to fall to around $6 a tonne.

The modelling found that as the cheapest forms of carbon abatement were used, the Coalition's scheme would have to pay about $58 a tonne to achieve Australia's full target. The Coalition has said it doesn't think it will have to pay more than about $15 a tonne.

The Coalition has not done its own modelling of Direct Action, but dismissed the Sinclair Knight Merz/MMA, with Coalition leader Tony Abbott saying: "I simply don't accept the report."

But the broad conclusions of the Reputex and Sinclair Knight/MMR modelling is consistent with several previous analyses, including by Ernst & Young, Allens and the Treasury.

Reputex also points out that the Coalition's intention to run "reverse auctions" to decide which projects get the government subsidies to reduce their emissions makes it difficult for the government and for companies to predict or plan for the outcomes.

"It is intended that funding through the emissions reduction fund will be disbursed through a series of reverse auctions, whereby companies bid to deliver abatement projects and lowest cost projects win the auction tender," the report says.

"This means funding for emissions abatement will be dependent on the outcome of successive auctions conducted over time, and not guaranteed prior to commencement.

"The risk is that reverse auction schemes alone do not provide enough capital certainty to developers, making project planning and delivery difficult and expensive."

In the leaders' debate on Wednesday night, Abbott said a strong economy could help deliver a better environment.

"If we have a stronger economy we are much more likely to be able to have the environmental safeguards that we want in order to ensure that the environment we give to our kids and grandkids is better than the one we inherited," he said.

"Poor countries tend to have much worse environmental records than relatively rich ones, so I think a stronger economy does go hand in hand with a better environment."