He won the 3000m with a personal best of 8m 35.11s, which sliced nearly 18 seconds off his previous best.
He and pre-race favourite Murdoch McIntyre (Auckland) broke away from the field, with Back kicking for home 300m out amd following up with another 50m burst that was not responded to.
Back, a relative novice, won the steeples on Sunday morning with an impressive performance and another personal best (6m 11.94s).
Around 150 minutes later, Back's younger training partner George Lambert took bronze in his national championships debut as Back returned to the track to win the 1500m in a very different sort of race.
It was slow and very tactical and as so often in championship races there was considerable jostling.
Back made his victory bid with 220m to go in a final lap, when the pace had finally been injected.
His final sprint saw off the challengers, as Back was timed at 55 seconds over the final lap.
Not long after, he needed all this speed as he ran in the 4x400m relay, taking the anchor leg in a personal-best 52 second lap.
Travis Bayler, who had earlier recorded a personal best 50.88s for the one lap to finish fourth in his debut nationals, likewise ran well in that Manawatu-Wanganui silver medal-winning team.
Genna Maples, 14, also returned with four medals – three individual and one relay.
Maples won the 100m (12.32s), backing up her NZ Schools win of December.
She also won the 200m, beating the Cantabrian Mia Broughton, who had finished ahead of Maples in December in an excellent 25.42s into a stiff 2.9m/s head wind.
Maples was second in the Long Jump with a calendar year best of 5.56m, and anchored both the 4x100 and 4x400 relays.
The former was with Tayla Brunger, Sophie Redmayne, and Olivia Seymour in which they had to run in the Senior Women competition, finishing in fourth, but with the consolation of a Wanganui Collegiate record of 48.86s – eclipsing the 10-year-old record by nearly half a second.
Maples had to dig deep in the 4x400m relay to take gold on the line after a thrilling Under 18 final.
Maples was clocked at 58.1s for her leg, following great runs from Sophie Redmayne in third, while young Ana Brabyn – also still only 14 – was running second after Tayla Brunger took the first leg.
Brabyn and Redmayne both ran under 60 seconds for the first time.
The winning time of 3m 56.64s broke the Wanganui Collegiate record by 0.04 seconds, set at the same championships a year ago.
The win came from a entire school team, competing against strong regional combinations.
Brunger had a day earlier won the 400m Under 18 title in convincing fashion with a big personal best of 56.26s, which set a new Wanganui Collegiate record after an outstanding controlled race.
She also took 200m bronze in the race won by her teammate Maples.
The other local medal came in the Under 18's 300m hurdles where Waikato-registered Joseph Sinclair, now at Collegiate, took the bronze after also finishing fourth in the 800m with his first sub-2 minute 800m personal bests, in both heat and final.
Lexi Maples, on the eve of her departure for Christchurch, came so close to a bronze medal in the Javelin, losing out by 10cm in the final round which would have been very much consolation following injury in the Heptathlon a fortnight earlier.
