He is strictly a 3D artist, focusing on assemblage and modelling works, as well as some carvings.
The most striking piece in the exhibition is "Mowa" - a large moa made of ... mower parts.
"Except for a few connecting pieces, every single piece in the moa is made of old mower parts," Mr Macmillan said.
"There is usually a story behind the piece and the parts it's made from. For example, I made a praying mantis - praying mantises are found in the garden - I made it from old garden tools."
There's also an eagle's head made of nine different types of saw blades.
Mr Macmillan said he often doesn't know what a work will be when he starts making it, as his artistic skills don't include sketching.
"I usually start with a Plan A, but that doesn't always work. It's about playing with the materials and letting them speak to you."
Also included in the exhibition are small cast bronze works of skinks, turtles, frogs and the like. These reflect Mr Macmillan's love of the natural world.
Next year Mr Macmillan will be taking part in Artist Open Studios Whanganui for the first time, as part of a collective who will be exhibiting from the Whanganui Resource Recovery Centre.
The exhibition closes on November 27.