"In the last four months, South Africa has dumped almost 300 000 metric tons of GM maize on to Kenya, Mozambique and Swaziland," it said in a statement.
This was revealed in its study, "Good neighbour? South Africa forcing GM maize onto African markets and policy makers", ACB said.
The study contained the first documented cases of commodity exports of genetically modified organisms (GMOs) from South Africa to the rest of the African continent, it said, describing this as a "a worrying precedent".
The ACB said that even Kenya, which was strategically important to Africa's biotech industry, and where most of the GM maize ended up, lacked the capacity to ensure the safe handling and monitoring of such a huge volume.
ACB director Mariam Mayet said as much as 80% of the grain trade in East Africa was informal and undocumented.
"The arrival of 280 000 tons of GM maize into Kenya presents the potential for genetic contamination on an unimaginable scale," she said.
"These shipments have come at a time when South Africa has experienced its second largest maize harvest on record, at over 13 million tons."
She said the latest available figures from the United Nations indicated that improved maize production in Kenya and Mozambique would result in increased food security in the two countries in the coming year.
"In both cases, analysts have stated that any remaining shortages can be plugged by inter-regional trade and government policies, without the need to impose GMOs on countries that have no infrastructure to safely handle them."
No bio-safety legislation
She said none of the three countries had functional biosafety legislation in place, let alone monitoring capacities.
"Swaziland has already refused entry to two shipments of GM maize this year, but records on the department of agriculture's website indicate that the South African response was to merely re-issue the permits to agribusiness, showing a flagrant disregard to its neighbour's right to say no."
Mayet said her organisation condemned "the underhand tactics and dubious motives" behind these GMO shipments.
ACB demanded that the South African GMO authorities conduct their affairs transparently, in a way that respected the country's neighbours "in keeping with the original intentions of the Cartagena Protocol on Biosafety".