Baron, who corners for Hopman, said the young fighter had fought flawlessly to come away with a unanimous points decision on Saturday after "throwing everything" at his opponent, who barely withstood the onslaught.
"McGrath has extremely heavy hands and undeniable knockout power but Zane was far too slick and just took him apart basically.
"He did everything right and showed just what a class act he is. Zane is the best cruiserweight out there and on Saturday he demonstrated why he is New Zealand champion."
In 2006 Baron won the same title, which he successfully defended eight times before injury forced him to relinquish the championship belt in 2010.
Hopman said he had kept strictly to his fight plan to disrupt McGrath and any advances he tried to make, and felt strong and comfortable throughout all five rounds of the bout.
Cornering for McGrath on Saturday night was six-time World Champion heavyweight kickboxer Jason 'Psycho' Suttie, who co-owns Elite Thai Kickboxing in the city of sails.
Hopman said he had spoken with Suttie after his victory about fighting at the televised semi-pro King of the Ring events that Suttie organises.
"This is as good as it gets for an amateur in New Zealand and after speaking with Jason I'm definitely confident I can get on the King of the Ring card," Hopman said.
Fellow Fortitude fighter Brent Segrue also took a win after fighting a 72kg first-time contest bout against Assassin's Guild Wanganui fighter Jesse Johnston, who he knocked down twice in the second round before taking a TKO win after a third knockdown in the third round.
Sixteen-year-old Fortitude fighter Jade Williams had a split decision go against him in his 60kg bout on Saturday against Tyler Hedley, of New Plymouth's Thai Boxing Studio.
Both fighters were undefeated going in to the bout and while Hedley took the first round, Williams fought back to take the second and lost on a split decision in the third, during which the referee misjudged the end of the bout and halted Williams during an end of round flurry.
Alexis Serepisos, of JAI Thai Boxing Gym in Wellington, had successfully defended his WMC North Island Lightweight title on Saturday night after taking the fight by TKO over Sanchi Aung, of City Kickboxing in Auckland.
Baron and wife Kylie had run the previous Battle of the Bush fights, although the event on Saturday was the first hosted by Fortitude and the first tournament held at the Genesis Energy Recreation Centre, he said.
There had been about 1000 fans at the 13-fight tournament, Baron said, which was an "outright success" he credited to Fortitude members and supporters and the major sponsors including Wai Weight Private Gym Masterton, The Rock FM, Sargents Motorcycles, PGG Wrightsons, Pinfolds Transport, Supreme Automotive, Ordish and Stevens, Bill Hammond Transport, and Hair EFX.