With bass player Hayden McFie, who has since left school, Half Eaten Pie won the 2016 and 2017 Waikato event and made the national finals.
Suggested Serving Size placed third at Waikato, so it came as a complete surprise when they were announced as national finalists.
Top bands from the regional finals submitted 'audition' videos to be judged for the grand final.
Lachlan says it was an honour to make it through for the third year.
"There were some great bands this year."
Playing as a duo, but categorised a band because they use a drum kit, Lachlan and Matthew wrote new songs for the contest.
Lachlan added an octave pedal to his kit and can drop his guitar down to give a rounder sound usually provided by bass.
Otherwise they haven't changed their style much.
Suggested Serving Size also never played live until they hit the Smokefreerockquest stage.
Half Eaten Pie still plays live, so they are including the new material as rehearsal for the final.
The pair say while winning would be great, the real excitement is being in the final again and playing to such a big and appreciative crowd.
"We get to meet other great musicians our age and that networking leads to chances to perform with the amazing bands," says Lachlan.
"That is the main takeaway from the final."
The boys are also looking forward to working at Neil Finn's Roundhead Studios again as well as part of the deal.
Te Awamutu College principal Tony Membery says the school is proud of Lachlan and Matthew.
"It can't be easy, but they've proven they have the drive and talent to perform consistently at a high level," he says.
"Well done to them."
Half Eaten Pie has a six track EP available now on Spotify. Suggested Serving Size audition videos are viewable by searching the band name on YouTube.
