Both now head to the world championships in England next month, where they are strong contenders in their respective categories.
Meanwhile Jenna Hastings, in her first nationals, took out the 8-year-old girls' title with Rotorua clubmate Megan Williams, also in her first national competition, third.
Lachlan's younger brother Wyatt was second in the 5-6-year-old boys behind Lucas Bhimy from North Harbour. Mrs Stevens-McNabb said the two boys were good friends who would renew their rivalry at the world champs.
Tuhoto-Ariki Pene capped his selection in the New Zealand Mighty 11 side to take on Australia with second place in the 11-year-old boys' final.
Rotorua riders featured strongly in the finals fields of what was the biggest national championships.
New Plymouth pair Matt Cameron and Victoria Hill took out the elite men's and women's titles on Saturday.
Cameron is a first-time winner in the absence of the country's top three riders, Marc Willers, Kurt Pickard and Kurt James who are in Europe preparing for the next round of the World Cup Supercross series in Norway this weekend.
"After 10 years in the sport I really wanted this, after being second several times," Cameron said.
He had to come from behind on the final straight to better Christchurch's Daniel Franks.
Hill, who has won at age group level in the past did not have things all her own way either, battling with 17-year-old Monique Smith of Gisborne.
Trent Jones from Kaiapoi took out the junior men's title comfortably.
Cameron and Hill repeated their victories in yesterday's superclass, with Cody Stevens and Nick Fox taking the minor places in the men's event.
