Supporters of Protect Karangahake, an organisation set up to prevent mining in the gorge, have vowed to disrupt activity to protect the mountain and its diverse ecology.
Protester Brian Habberfield, of Waikino, said the letter essentially said the mining companies were not welcome in the area and they were opposed to what they were doing to the mountain.
"Karangahake is an iconic piece of geography and it's a hugely popular recreational area.
"The ecology is still recovering from the old mining."
The strip where mining would occur was relatively narrow, he said, and as an ecological corridor from the Kaimai Mamaku to the Coromandel it was recognised as a significant natural area.
He said the noise that would be involved in mining operations would upset wildlife such as birds and bats.
Mr Habberfield said mining companies could never guarantee things would not go wrong.
"They always come in with assurances about how nowadays it's so safe and they've got the engineering," he said.
"We're not going away.
"I hope people will be alerted to what's going on in their beautiful backyard."
Newcrest Mining, which was providing technical direction and funding for the drilling, was exploring two areas in the Karangahake Gorge: Waitekauri and Owharoa.
A Newcrest spokesman said the drilling was not located on conservation land.
"Environmental management plans are developed to identify and outline controls to prevent potential environmental impacts during the establishment of the drill site and during drilling activities.
"All drill sites will be fully rehabilitated once drilling has been completed."
