At one point, horizontal rain and fierce crosswinds blew riders across the road. Four riders abandoned the race.
Conditions also caused the peloton to fracture several times.
A lead group of nine split into three, which decided the top-three placings, Mould winning from Jordan Kerby, with Cooper third.
Gordon was officially 83rd. He came in with the main peloton, whose riders were 19 minutes 40 seconds behind the top three.
Cooper’s efforts brought him the yellow jersey as overall race leader, 36s ahead of Griffin, with New Zealand National Team member James Oram — the previous race leader — another second back.
Gordon is 39th overall, 24m 52s adrift of Cooper.
The classic finishes today with a 120km stage, comprising 12 laps of a 10-lap circuit on the outskirts of Masterton.