"Lee has been responsible for disseminating extreme white supremacist material, and has consistently been reported for creating objectionable and racist media for his channel," it reads.
"More recently he has taken to directly attacking Maori MPs, Rawiri Waititi and Hon Willie Jackson, using blatantly racist rhetoric and is actively and deliberately stoking public fear in response to the He Puapua report."
Videos cited by Newshub backed this up, including putting on a fake 'Maori' accent to impersonate Waititi.
Synlait told Newshub it was "deeply concerned" and "takes these matters very seriously", so was investigating.
"We are deeply concerned that someone has these views. Lee Williams' views are his own. These views are not shared or held by Synlait," a spokesperson said.