Phelan's judgement in letting the Harvey Wilson-trained 9yr-old roll along at the head of the field proved perfect when It's A Wonder ran away to win the 5600m feature by 23 lengths on a rain-soaked Riccarton track.
The winning duo led by about 10 lengths in the middle stages.
While he was out of his own with It's A Wonder, Phelan resorted to singing to himself to help his concentration.
"She was lonely out there in front and I think I sang my favourite song, Blurred Lines, about three or four times," the jockey said.
"I just thought I'd let him do what he does best which is to roll along."
It's A Wonder's victory sealed a Grand National carnival double after he won the Koral Steeplechase seven days earlier.
"Harvey is one of the best jumps trainers but he's never really had a good go at one of these big races. He's found the right horse to do it today," Phelan said.
"He's such a great conditioner of a horse.
"That's why I rode him how I did as I knew he would be the fittest horse in the race."
Saturday's meeting was run in pouring rain with mud splattering runners in each of the 10 races.
Wilson admitted he was hoping for a postponement of the day's racing as he had major concerns about how his horse would handle the deep and testing heavy 11 track.
"We were actually hoping they might put the races off as we weren't confident at all with this rain," he said.
"Shaun was fantastic. He [It's A Wonder] jumped fantastic, what a thrill. This is as good it gets."
Wilson, who competed at the 1988 and 1992 Olympics in equestrian, produced Bad Boy Brown to run second in the Sydenham Hurdles and the Grand National Hurdles during his brilliant week in Christchurch.
Phelan's tactics on It's A Wonder meant his rivals were effectively in a race of their own, running for second.
Gagarin was second ahead of Mr Enthusiastic.
