Despite the innovative format, the 39-year-old Australian put together what was effectively a wire-to-wire victory at the Lake Karrinyup Country Club, having finished the first three rounds of strokeplay with a five-shot lead.
The final was all square until the fourth hole when Rumford somehow got himself out of the trees, over a bunker and onto the green with a brilliant second shot before sinking a 10-foot putt to take the lead.
At the par-three penultimate hole, Rumford hit his tee shot to within three feet of the flag, while Phachara found the greenside bunker and had no luck putting his way out of the sand to effectively concede the contest.
"It's amazing, particularly with the new format, the first win in that," said Rumford.
"It's been a long, hard-fought week (and) I'm really proud of myself."
It was a sixth European Tour victory for Rumford, who turned professional the year after Phachara was born, and re-secured him the tour rights he lost at the end of last year.
"It's massive," he added. "I had a tough year but I'm back and I couldn't be happier."
While Rumford dominated all four days of the tournament, Phachara had trailed by nine shots after round three and grabbed the 24th and final spot in Sunday's matchplay knockout series with a par at the third playoff hole.
The 17-year-old was on fire on Sunday, however, and wins over locals Sam Brazel, Lucas Herbert, Matt Millar and Jason Scrivener put him into the final of the event, also co-sanctioned by the Australasian PGA and Asian Tours.
He faced a long wait as Rumford battled his way past Adam Bland in the semi-finals and his touch ultimately deserted him at the end of a long day in the Perth sunshine.
Asked whether it was fair that a player nine shots off the pace after the third round should be in the hunt for the title at the end of day four, Rumford, the trophy safely in his hands, said he was happy with anything that was good for the sport.
"It really comes down to the people and golf is bigger than one person," he said.
"If it's going to grow the game ... and brings a bit more excitement and thrill, if that's what we've created, then I think we've done a great job."
Bland, who knocked out former British Open champion Louis Oosthuizen in the quarter-finals, finished third after beating Scrivener 1-up in a playoff.
South African Oosthuizen had to settle for fifth place after seeing off Australian Steven Jeffress 1-up in the fifth-sixth place playoff.
(Reporting by Nick Mulvenney in Sydney, editing by Amlan Chakraborty)
