South Australian farmers are now allowed to buy and grow genetically modified seeds and crops, after a 15-year long moratorium was lifted by the state government.

New regulations around GM crops took effect on Wednesday - the first day of 2020 - across the entire state except Kangaroo Island.

"By lifting the GM moratorium on mainland South Australia today, we are backing our farmers and researchers to grow the state's agriculture sector and create jobs," Primary Industries Minister Tim Whetstone said.

"For 15 long years South Australia has suffered under a moratorium which has cost our grain growers real money, restricted our scientists' ability to conduct world-leading research and put a handbrake on our state's economy."

But the move could be overturned when state parliament returns in February, with Greens MLC Mark Parnell vowing to bring a disallowance motion.

Labor and SA-Best have twice blocked the government's attempts to remove the ban.

Mr Whetstone urged them to "stop getting in the way of progress" and abandon plans to reinstate it.

"The decision to lift the GM moratorium followed a high-level independent expert review, extensive industry and community consultation and the recommendations of the GM Crop Advisory Committee," he said.

He said the review found the moratorium has cost the state's grain growers at least $33 million since 2004, and would cost at least a further $5 million if extended to 2025.

South Australia is the last mainland state to lift its GM moratorium, which was introduced by the Labor government in 2003.