Mr Macdonald said the plant was about 10 years old, and had flourished particularly in the past five years. It lives in a shade house - a location that Mr Macdonald said must suit it.
"The important thing about it is that it's evenly covered with flowers - there are no gaps. And I've done my best to keep the leaves in good condition so there are no black marks on them."
Mr Macdonald, who has been growing orchids for about 25 years, was originally from Whanganui, and said he was delighted to win grand champion at his old stomping ground.
Wanganui Orchid Society's president Kath Bourne said the show had been extremely popular, with people lined up waiting for the doors to open on Saturday morning, and a steady stream of people all weekend.
It was the Wanganui Orchid Society's 50th annual show. The special occasion had drawn growers and enthusiasts from throughout the North Island, from Wellington to Auckland.
"The judges said it was one of the best shows they'd seen," Mrs Bourne said.