Almost on halftime, Levin came close to pulling one back. Skipper and central defender Connor Vine spotted a gap in the Thistle rearguard and sprinted 40 metres to capitalise. The Levin midfield got the ball to him, in the clear on the right.
However, he was pushed wide by the pursuing cover defence, and his powerfully hit shot was just outside the far post.
Somerton continued to cause Levin problems in the second half. In the 48th minute he made a dangerous run down the right but was foiled by a sound covering tackle by centreback Jesse Skinner.
In the 57th, Somerton made ground on the left and cut the ball back, just behind his strike partner Alan Zarate.
Three minutes later, Somerton was shown the yellow card for clipping the heels of an opponent making his way through the centre-circle to start an attack. Earlier, in the 35th, Somerton had drawn a shunting foul that earned Levin's Skinner a yellow card.
Spawforth's 71st-minute free-kick, awarded for a foul on centreback Ander Batarrita as he made a foray into the attacking third, was a pearler. From 27 metres, to the left of the goal, he hit the ball over the wall and into the far corner.
A minute later, Levin midfielder Bayly Vine was sinbinned for 10 minutes for dissent.
A man up, Thistle seemed content to keep possession without pushing hard to increase their lead. But in a late rush they scored in the 87th minute, second-half substitute Travis White scoring his first goal at this level. Somerton had intercepted a clearing kick by goalkeeper Bayley Butler-Easton and passed for White to score with a first-time right-footed shot.
Somerton almost completed a hat-trick in the last minute, but midfielder Sam Larmer did well to get back and block the shot.
Levin coach Peter Vine was pleased with the performance of his side. His eldest son Connor was, at 25, the oldest player in the team.
“The rest are 22 down to 18,” he said.
“Normally we'd have a 17-year-old, too, but he was injured in last week's game.”
He thought Levin did well for a town of around 19,000 to be able to field a home-grown team who played good football and held their own at Federation League level. But they hadn't been scoring enough goals to climb the table.
He had three sons and a nephew in the team and two of them, nephew Joshua Vine in midfield and son Connor at the back, were among Levin's best.
Goalkeeper Butler-Easton, defenders Skinner and Neil Auckram, and midfielder Larmer also put in good shifts.
Thistle coach Garrett Blair said Levin were a well-structured side who knocked the ball around nicely. At halftime the message had been for his side to give Levin's players less time on the ball.
He thought Thistle put in a good team performance, but he was particularly impressed with Daniel Venema's composure at leftback.
On attack, Somerton was always a threat, Zarate worked hard and held the ball up well, and Brouwer was quick to put pressure on defenders.
The game was well controlled by referee Chris Niven.
Thistle play North End, of Palmerston North, in Gisborne on Saturday, and end their league campaign against Havelock North Wanderers in Hawke's Bay on August 14.
Other results: North End 5 New Plymouth Rangers 1, Havelock North Wanderers 2 Massey University 1, Whanganui Athletic 2 Napier Marist 0.