Environmental groups and businesses connected with electric cars have called for the vehicles to be exempt from stamp duty if a tax on how many kilometres they drive passes South Australian Parliament.

The state government has already announced a $3,000 subsidy for the first 6,000 electric cars sold each year — but it only plans to introduce the discount if a road user charge on kilometres driven, announced in last year's state budget, is also passed.

Waiving stamp duty on electric cars would save buyers between $1,500 and $8,000 depending on the value of their purchase.

Signatories to the letter to state politicians include Mitsubishi, Volkswagen, the Electric Vehicle Council, Solar Citizens, Conservation SA, Doctors for the Environment Australia and The Australia Institute.

As well as calling for a stamp duty waiver, they also want the planned subsidy implemented.

The Australia Institute's South Australian director, Noah Schultz-Byard, said a survey had found seven out of 10 South Australians would be less likely to buy an electric vehicle (EV) if the government introduced the road user charge.

The government has put off introducing the tax until 2027 or when electric vehicles make up 30 per cent of all new vehicle sales.

It would eventually help recuperate lower tax collected from the fuel tax excise as more people turn to electric cars.

The proposed subsidy would come in on July 1 next year, after the March state election, and would cost the government $36 million.

Government promotes backers

Following the publication of the letter, the state government this afternoon promoted support of its policy from the Royal Automobile Association of South Australia (RAA), the Federal Chamber of Automotive Industries and Business SA.

"The delay of the road user charge, along with an incentive package, should support EV uptake and ensure we have a sustainable transport funding model going forward," RAA mobility technology specialist Mark Borlace said.

Treasurer Rob Lucas said it was important there was a "long-term sustainable model for critical road funding".

"They know, as the state transitions towards a higher concentration of zero and low emission vehicles, it's vital to ensure all vehicle owners, regardless of what car they drive, contribute to the upkeep of our roads into the future," he said.

The Opposition and crossbenchers oppose a road user charge.

"Car manufacturers and groups see there is a future in this industry, and they want the Marshall Liberal government to back them in — not enforce more taxes," Labor environment spokeswoman Susan Close said.

The Victorian, NSW and Tasmanian government have all announced road user charges following the South Australian government's lead.

The Victorian incentive program is similar to South Australia, with a $3,000 subsidy offered on cheaper electric vehicles.

NSW has promised stamp duty waivers and a $3,000 rebate, while Tasmania has promised a two-year stamp duty waiver.