Great project to support – in addition to the great suggestions others have provided perhaps connect also with Volunteering New Zealand – they may be able to provide you with some further channels / suggestions to assist you as well as perhaps helping you with connecting with active Volunteers registered with them.
Best of luck!
“The glory of gardening: hands in the dirt, head in the sun, heart with nature. To nurture a garden is to feed not just the body, but the soul”.
— Alfred Austin