Three cannabis plantations have been discovered in a coastal national park in New South Wales.

Hundreds of plants with an estimated street value of $900,000 were found at three sites in Yuraygir National Park, east of Grafton, police said.

"We've seized over 560 cannabis plants... that crop is worth about $900,000 on the street," Detective Inspector Darren James said.

"It was seized from three separate crop locations west of Glenreagh.

"Our investigations are ongoing in relation to who were operating that cannabis plantation, and those inquiries will continue."

No arrests have been made.