An $18 million crop of cannabis has been found growing in a national park on the NSW Mid North Coast.

More than 9000 plants were growing on two leased properties in the remote Chaelundi National Park, about halfway between Grafton and Dorrigo.

On Wednesday morning police officers from Strike Force Morison, formed to investigate the matter, found two men aged 61 and 37 watering the crop, police said.

Police raided the property and arrested the men, as well as one other man, aged 52, who was found on a track leading into the property.

The 37-year-old and 61-year-old were charged with cultivating cannabis greater than a commercial quantity.

They were refused bail in court yesterday and will reappear at Coffs Harbour Local Court on March 9.

The 52-year-old was charged with revocation of a parole warrant.

The national park holds the largest stand of old-growth forest in NSW, the State Government says.