A meeting between Dr O'Sullivan's supporters and members of the Te Hauora o Te Hiku o Te Ika board was scheduled to take place at Mahimaru Marae on Monday.
Waireti Walters, a patient of Dr O'Sullivan for six years, said she felt "betrayed and ambushed".
"Lance has made a point of coming home to look after his people ... The worst part is that these organisations were put in place to help Maori health, and they ain't doing that."
Te Hauora o Te Hiku o Te Ika Trust chairman Bruce Gregory said Dr O'Sullivan's resignation had been accepted with regret. He would not officially leave the practice until August 10, but in the interim was taking annual and sabbatical leave that had been approved earlier. The trust had asked Dr O'Sullivan to reconsider his resignation but he had declined to do so.
"His decision is his own, and we thank him for his valuable service to the Far North over many years," Dr Gregory said.
The trust would ensure his patients continued to be well cared for.