AFFCO Wairoa Meat Workers Union president Dale Robinson said the letter was designed to demoralise workers.
"We know we've got the support of the community to keep us strong," he said.
Ngati Kahungunu Iwi Incorporated chairman Ngahiwi Tomoana said Maori farmers would be asked to not send their stock to Affco and offshore buyers asked to boycott Affco products.
Syd Keepa, CTU Vice President Maori yesterday welcomed the iwi's support and their stock-witholding strategy.
"We also want to acknowledge the work the iwi is doing to support the locked out workers and their whanau. The support centre opened by Ngati Kahungunu in Wairoa on Monday provides much appreciated food and other services from the community to the locked out workers and their families.
"We're behind these workers one hundred per cent, and there is widespread support for AFFCO workers in the union movement generally.
"We are calling on AFFCO to end the lockout and return to bargaining."