"It is fantastic especially under the circumstances – starting off the second, a good start and the car was great," he said after the race.
In terms of the championship lead, Armstrong didn't want to even think about it.
"It is good but there are four more rounds to go so it is a little irrelevant," he said.
Armstrong's FDA colleague Robert Shwartzman secured second place – his third successive podium - while Brit James Pull was third.
Southlander Brendon Leitch finished fourth having battled Pull throughout the race but just couldn't find a way past. It salvaged some points for the 21-year-old, who struck trouble in both of the other races across the weekend.
The other New Zealanders weren't as strong – Ryan Yardley was eighth, Reid Harker ninth and Taylor Cockerton 12th.
Harker, who had impressed in his first two races in the series this weekend, couldn't get his car off the line and it brought out a safety car on the opening lap. Yardley, last summer's Toyota 86 champion, was steady all weekend without setting the world on fire while Cockerton had been competitive in the first two races but found himself at the back of the pack in the feature.
Dutchman Richard Verschoor, who won the Saturday race and finished second in race two, saw his luck desert him when he pulled from the race track while running second with an electrical issue.
Verschoor's misfortune leaves Armstrong in command of the standings. The Kiwi holds a 16-point lead over Shwartzman with Verschoor in third – 52 points further adrift.
The series moves to Teretonga, near Invercargill next weekend.
Championship standings
Marcus Armstrong 210
Robert Shwartzman 194
Richard Verschoor 142
