Kiwi fashion designer Dame Trelise Cooper has asked people who doubt the ethics of her company's manufacturing practices to unfollow her social media accounts, after facing flak from customers for receiving an 'F' in Tearfund's ethical report.

Cooper has long criticised Tearfund's ethical fashion report that grades brands from A to F on a number of criteria, including environmental management, worker empowerment and transparency, for being misleading and deceptive because her company opted not to participate.

Cooper's eponymous clothing brand issued a statement on Wednesday saying there were "two ways that you can obtain an F through the Tearfund reporting practices: Non-participation in the report – this applies to Trelise Cooper Group; or Tearfund judging a business's practices are unethical, non-compliant and not socially responsible – this does not apply to Trelise Cooper Group."

Cooper told Stuff her company was "sincerely dedicated to strong ethical standards" but hadn't the man-power to spend four months collecting data for the detailed report that Tearfund demands each year.

On Facebook, hit back at comments questioning her company's ethics asking people to "remove yourself from our social media community" if the company's 35 years of "good businesses" was not sufficient.

The company posted: "Integrity, honesty and generosity are the heart of our business. We care deeply about the people involved in all areas of our business and work tireless to ensure all people involved are respected and fairly treated."

"If our word, integrity and 35 years of good business is not sufficient to reassure you of our practices and integrity please feel free to remove yourselves from our social media community."

Tearfund's education and advocacy manager Claire Hart said the fashion industry caused significant environmental degradation "which affects the wellbeing of workers, the community and their natural environment".

"Through assessing their materials and facilities, brands can take informed steps to reduce their environmental impact – from the farm to the final product."

Tearfund said they believed their four-month time-frame was a generous window to collect the information needed. If they then aren't handed the data, they do an online search and grade from there. They said there was no automatic F awarded or non-participation.

But it's this online search, followed by a potentially damning verdict, which is ruffling feathers.

Kiwi designer Karen Walker might have got a C, but she also slammed the survey as "inconsistent and unreliable in many ways" last year.

Meanwhile, Aussie brand Pavement, who were given an F this year following a D in the last report, have put a statement on their website. "As a relatively small business with very limited resources, we simply have not had the time or money to dedicate to pulling together this documentation."

Many in the ethical sin-bin are distributors to larger companies – like H-Wood shoes (D+) and Double Oaks Mills (D+) who sell through The Iconic – and don't have their own website from which Tearfund can source information.

Kiwi designer and retailer Annah Stretton said ethical fashion was currently a huge challenge for many mid-range brands and rating them an A to F wasn't that helpful in what she calls a "disruptive industry".

"At the very core, ethical sourcing is very important to all of us, but the realities of being able to do that and afford to do that isn't always viable," says Stretton.

"As a New Zealand label, you are so small in the scheme of things. It's very hard to start stipulating that you want ensure that fabrics, buttons, threads and zips  used are free from the conditions that Tearfund try to eliminate and, even if you are promised, it's very hard to track this."

Jennifer Whitty, senior lecturer at Massey University's School of Design, has been working hard to find a solution to the damage that throwaway fashion has caused. But she says that while Tearfund's aim for a transparent industry is a positive, their method is flawed.

"This topic is complex and the territory is changing all the time with new information and innovations. We need to be careful not to oversimplify the issue and reduce 'sustainable fashion' to a tick- box exercise or vilify brands as bad."