"The entire squad worked well on defence in game one and that was the key to our win," co-coach Tom Blake said.
Havelock North's second pool game opponents, Feilding Yellow, were one of two teams who withdrew late from the 16-team event.
Havelock North finished their campaign with a 45-0 drubbing of Canterbury champions Saracens.
Flanders and the versatile Jesse Paewai scored two tries each. Blake, Spooner-Neera and Anzelo Tuitavuki scored the others. Blake kicked four conversions and Matt Hill the other.
MAC lost 17-7 to Ardmore Marist in their first outing. MAC led 7-5 at halftime after versatile back Siave Togoiu scored during the first 30 seconds of the game.
Togoiu, who was part of the Warriors NRL training squad last year, was outstanding for MAC when they won the Napier Old Boys Marist Invitation Sevens tournament in November.
Former Samoa Sevens representative Tom Iosefo was another to impress in the MAC backline during this encounter while Willie Benson, a New Zealand swimming representative at the 2008 Beijing Olympics, had a huge workrate in the MAC engine room.
"We didn't use our possession wisely. Then we ended up not having much and you can't afford to be in that position at this level," MAC coach Anthony Morley said.
In their second pool game MAC conceded a late converted try to lose 26-21 to Wellington champions Hutt City Old Boys.
"Both of our first two games were there for the taking. Our boys know they're at the nationals ... you are punished for any little mistake at this level," Morley said.
Sweeper, Alatasi Tupou, Iosefo and substitute winger Roger Paewai impressed in the MAC backline. Tiki Greening shone with his aerial play and fellow prop Teina Huia provided some valuable physicality.
Paewai, Iosefo and Togoiu scored MAC's tries and Tupou kicked three conversions. MAC will start Saturday's play in the Bowl division after losing their final outing of the day 22-12 to unbeaten pool winners Te Puna of Bay of Plenty.
Benson was the pick of the MAC players in this match and scored a run-away try. Tupou scored the other try and kicked a conversion.
Iosefo shone with his impact off the bench and hooker Vai Kereti had his best match of the day.
This weekend's 14 teams arrived in the capital with the aim of tasting Middlesex County Wavell Wakefield Cup glory.
The cup was brought to New Zealand by the manager of the British Lions, L.B. (Ginger) Osborne, in 1950, and the tournament began in Dunedin in 1951.
It was first won by Otago club Zingari-Richmond. Interest in the tournament declined in the 1990s and wasn't contested for a decade up to its revival in 2006.
In 68 years of competition, it has been held by 28 teams, 11 from the North Island and 17 from the South Island. It had been held by South Island clubs continuously for more than three decades before Auckland's Pakuranga United won in 2013 to become the first North Island team to prevail since Hamilton Old Boys in 1976.
This weekend's hosts Northern United won the tournament last year after Auckland club Eden had won it in 2017 and 2018. No Auckland clubs entered this weekend's tournament.
When this tournament was last held in Wellington in 2016, Waikato's Melville won with the help of now All Blacks winger Sevu Reece who was stepping into senior rugby for the first time since leaving Hamilton Boys' High School.