Mount's lead-out players quickly cut into the lead, with Steward's 35, Dave Enright (33), Owen Kendall's 35 and 15-year-old Bailey Smith's one-under 71, which yielded 37 points, all bankers.
No 1 Brad Kendall had 27, compounded by an out of bounds on the 18th, but Mark Hughes (33) and Campbell Irvine (35) did enough between them to maintain the lead and close to an eight-point win over Royal Auckland, with Akarana (380) third. Bay of Plenty's second team, Tauranga, were fifth.
Smith's round - 17 pars and birdie on the long par-four 16th - was the fourth-best of the day, with Akarana's Jono Ratcliffe hitting the day's low, a 68.
Smith, who attends Mt Maunganui College, said the win capped off a good run for the Mount, who beat Tauranga in Taupo to win the Bay of Plenty championship pennant championship.
"I've been working with Robert [coach Robert Rookes] on making birdies, and even though it didn't really work today I made some really good par saves," he said.
"Knowing the course helped too though because I knew breaks that the other guys weren't seeing."