The supermarket giant on Wednesday decided to provide its thick, reusable plastic bags for free indefinitely, instead of charging 15 cents each as part of its program to get shoppers to switch from using single-use plastic bags.

But after weathering a barrage of criticism during the past 24 hours, managing director John Durkan has told staff customers will have to start paying for the bags after August 29.

In a message to the retailer's 115,000 staff on Thursday, Mr Durkan said the ban on single-use plastic bags had been a "big and difficult" change for customers.

While customers had been growing more and more accustomed to bringing reusable bags, many were still finding themselves one or two short at the register.

"That's why we are extending our complimentary bag offer until Wednesday 29 August for our customers in Queensland, NSW, Victoria, and Western Australia," he said.

"I appreciate this transition phase is taking longer than anticipated but it is absolutely the right thing to do by our customers."

Environmental groups, including a vocal Greenpeace, and like-minded shoppers had heaped criticism on Coles for deciding to go back on its original plan to only temporarily provide reusable bags for free.

When Coles originally announced its ban on single-use plastic bags it said it would provide its reusable and thicker "Better Bags" for free until July 8 to help customers adjust.

It then extended the giveaway until August 1, but once that date rolled by walked away from the deadline and said the free bags would continue to be provided to shoppers.

Greenpeace spokeswoman Zoe Deans cautiously welcomed Coles' latest change of heart and the new August 29 deadline for the giveaway bags.

"It sounds like initially they paid too much attention to the vocal minority and I think the sheer scale of the outrage yesterday and this morning has shown them what Australians actually think," she told AAP.

"It's confusing and frustrating for customers that they have been flip-flopping on this issue and we really want to see them make a solid commitment to actually doing what they said they would do and ban the bags for good."

Ms Deans said given Coles had previously reneged on its deadlines to stop giving away plastic bags, some may be skeptical about them sticking to the new one.

"So it's up to all of us to keep the pressure on and tell them we want to stick to their commitment to reduce plastic," she said.

Mr Durkan said Coles was "absolutely committed" to reducing its impact on the environment by removing single-use plastic bags from stores.

Australian Associated Press