Mr Muir's children are now 13 and 17 years old and "intensely proud" of their dad after the weekend's win, he said.
"I'd always enjoyed building sandcastles but a few years back I found a book written by a professional sandcastle builder from Texas and suddenly got better."
Mr Muir said he believed his attention to detail helped him nab top prize, a package of camping and outdoor gear.
"People get really fascinated by it. It can sometimes take three to four hours to build one. I'll be building away and my daughter would say 'oh dad, there's 20 people watching you' and I turned around and just go 'wow'."
A few years ago, Mr Muir submitted a photo of a sandcastle he had built to a motoring "summer pictures" competition and won a Hyundai Santa Fe for a year.
"At that point, all of a sudden, you are vindicated. That gave me some credibility because until then people just thought I was nuts.
"This was an opportunity to see whether I was that good ... New Zealand has this phenomenal beach culture, everyone loves it, but no one builds castles - even at the Mount.
"I tend to do sandcastles because everyone can relate to them. If you're 5 years old it's the castle from Frozen, if you're 10, it's from Harry Potter. If you're 30 it's Game of Thrones and if you're older it's a historic castle from Germany.
"Sandcastles are quite iconic and timeless."