Kowhai Reach, in Kauana, Central Southland, is a rare ecosystem protected for more than 40 years by local farmers.
The planting has been organised by the Southland Ecological Restoration Network.
Queen Elizabeth II National Trust Southland representative Jesse Bythell said they had 600 trees to plant "so please tell all your friends, the more the merrier".
The aim of the supplementary planting was to give the forest a boost to ensure it had a long and healthy future.
The event would be held on Friday, October 27, between 10am and 1pm.
It was open to anyone who is interested, she said.
"Please let your catchment group co-ordinator or myself know if you intend to come so we can get a basic idea of numbers for the day."
Volunteers will need to bring their own spade, gloves, lunch and water. For more details visit www.sern.org.nz/events/
