The forest at Bushy Park Tarapuruhi will be extended outward from the sanctuary across a 2ha paddock, with the public invited to help in the planting.
Some planting has already been done by school groups, volunteers and visiting trampers.
The Bushy Park Trust has decided to retire the 2ha paddock reached by the loop of the Rātānui-Moore's Ride- Fenceline and Kapiti Tracks. Its eucalypts have been felled and it has views of Kāpiti Island on a clear day.
The paddock had been used to graze cattle while the sale of those cattle, many donated by local farmers, helps pay for sanctuary maintenance.
Paddocks near the Bushy Park Homestead are still grazed for this purpose, and the trust is grateful for the help.
Returning one paddock at the reserve to bush will provide more habitat for the sanctuary's protected wildlife - with "edge habitat" and variety.
Most of the trees and shrubs to be planted are sourced within the sanctuary, to maintain its genetics. However they will include non-native tree lucerne, Chamaecytisus palmensis, a short-lived small leguminous tree that attracts kererū (native pigeons).
The birds feed on tree lucerne flowers and leaves in winter. The trees shade out grass and feed soil, and seed from the pigeons' droppings grows new forest trees.
Forest is expected to take over from the tree lucerne within five to seven years, Brooke said.
The planting will also include harakeke (flax) from Parnells Quality Tree & Shrub Nursery. It will provide habitat for lizards, and native birds such as tūī, korimako (bellbird) and hihi will feed on nectar from the flowers.
The sanctuary is 25km from Whanganui on Rangitatau East Rd. Anyone who wants to help with planting is asked to be at the end of its Kāpiti Track, a 20 minute walk, at 11am on October 8. Sanctuary manager Mandy Brooke advises them to bring gumboots, gloves and a raincoat in case it rains.
If people want to stay longer it's a nice place to stop for a snack or a picnic, she said, and since it's the school holidays she's hoping families will come.