It was Toomey's second score of 58 after he set an unofficial record at the Hamilton Golf Club.
"I have been plugging away at a few things for a while and felt I had been close to a good score," he said.
"But no one steps on the first tee and thinks I'm going to shoot 58 today.
"I started birdie, birdie from number seven and then made a really good up and down on the par-3 number nine. I drove it on the 10th green and holed a 12 footer for eagle and the momentum just kept going from there really.
"Honestly it was not like I was striping it down every fairway and hitting it to within a foot on every hole.
"It was just really solid and I finally had a day when my putter got really hot so it was good."
Toomey admits he thought about breaking 60 later in his round.
"I must admit after I got through the 18th hole I thought we have a really good chance of breaking 60 here.
"I knew then I had to make three birdies in my last six holes.
"I was definitely thinking about it and holed my pitch shot on number two to make eagle and after that I thought this has got to be my day.
"It is always nice to sneak past your mentors like [Mark] Browney and get in ahead of him."
Toomey moved to Tauranga from Hamilton three weeks ago to work daily with his coach Jay Carter, based at the Tauranga course.
Carter has been coaching Toomey for several years and said he typified the modern high performance golfer.
"Obviously that 58 is impressive and he is talented and all that good stuff but he works so hard on every aspect of his game," Carter said.
"That is what a lot of people don't know about him. He is up at 5.30am every morning, he works hard at the gym and is the modern, athletic golfer.
"He is a pretty complete package."
Toomey will compete at the Japan Tour Q School next month as an amateur, with the world championships in September in Mexico likely to be his last major event before he turns professional.
