This glitch was quickly rectified and his charge back through the pack was inspirational, but ultimately cost him dearly too and, after finishing an unaccustomed fifth in that fateful race, he is now a huge 23 points adrift of Harwood with just three races at Taupō to complete the series.
Taupō's Wyatt Chase and Hamilton's Kayne Lamont are ranked third and fourth respectively after Saturday.
In the MX2 class, Purvis suffered his first setback of the series at Pukekohe. Unbeaten in this class after two rounds of the championship, he was twice forced to accept runner-up spots as Oparau's James Scott turned up the heat.
That may turn out to be little more than a judder bar on his way to successfully defending his MX2 crown. Purvis heads to Taupō with a massive 40-point advantage over second-ranked Scott.
It was a furious dog-fight in the MX125 class at Pukekohe, with the series leader at the start of the day, Tauranga's Madoc Dixon, dropping down to third in the standings, while Silverdale's Hayden Smith came on strong on Saturday to take over the championship lead.
Clevedon's Cobie Bourke was also outstanding at Pukekohe and now holds second spot outright. This remains the tightest class, with just 15 points separating the top three.
"Perhaps we are seeing a changing of the guard in the MX1 class," Motorcycling New Zealand motocross commissioner Ray Broad said.
"We have some exciting new talent coming through in this premier class, riders such as Te Puke's Tyler Steiner, Dunedin's Campbell King, Taupō's Hadleigh Knight and Napier's Tommy Watts impressing at Pukekohe.
"Maximus Purvis is looking good for a successful defence of his MX2 crown and he'll be off to race in Australia after the Taupō event ... we wish him well for that.
"The 125cc class is still wide open and these guys have been putting on a great show, which we are sure will carry on at Taupō."
The series wraps up with the fourth and final round at Taupō's Digger McEwen Motocross Park this Sunday.
Senior NZ Motocross Champs points after three of four rounds:
MX1 class: 1. West Auckland's Hamish Harwood, 192 points; 2. Mount Maunganui's Cody Cooper, 169; 3. Taupo's Wyatt Chase, 159.
MX2 class (provisional): 1. Mangakino's Maximus Purvis, 194 points; 2. Oparau's James Scott 154; 3. Silverdale's Hayden Smith, 151.
Under-19 class: 1. Matamata's Brodie Connolly, 150 points; 2. Te Aroha's Luke Van Der Lee, 145; 3. Christchurch's Marshall Phillips 128.
MX125 class: 1. Silverdale's Hayden Smith, 179; 2. Clevedon's Cobie Bourke, 172; 3. Tauranga's Madoc Dixon, 164.
MX3 class: 1. Hamilton's Darryll King, 137; 2 Whakatane's Darren Capill, 122; Tauranga's Brendon White, 85.