WGC responded to their worst away league result of the season two nights earlier, by equalling their most emphatic win of the campaign.
Not since the opening day of the season, when they triumphed 6-0 at London Tigers, have Adam Fisher's side won a game by a six goal margin.
Dave Parkinson, who has scored 13 in the last 16 WGC matches, was the Citizens' hat-trick hero, as hapless Harefield closed their season with this hefty defeat, which left them with an anxious wait to see if they will still be in the SSML Premier next season.
Relegation rivals Colney Heath needed a point from their final game to relegate Harefield.
At the other end of the table, the win lifted WGC to fourth in the table, ahead of League Trophy final victims Berkhamsted, on goal difference.
There was no sign of the second-half goal deluge in the opening 20 minutes of this encounter, with neither side creating a decent opening, until Parkinson exploded into action on 22 minutes, collecting a through ball, slipping inside a defender and striking a blistering 20 yarder into the top rigging.
That wasn't t all in keeping with the opening exchanges, but before a half-hour was up, WGC found themselves 2-0 to the good. A driven free-kick from Jay Welsh saw Harefield defender Clarkson, under pressure from Luke Robins, head the effort past his own keeper.
A very physcial Harefield responded and the huge, experienced Melford Simpson created a chance for himself that brought a diving save from Joe Croft before full-back Green struck a powerful, swerving effort past Croft and against a post, just before the interval, to show the visitors were still in the game.
Six minutes after the restart, WGC were 3-0 up, Parkinson slipped Ashley Kersey through. The big striker hurdled Murphy's attempted challenge and went clear to slam his effort past keeper Ballheimer.
Four minutes later it was four, as Parkinson was put clear down the left, cut inside and crashed another fierce effort over Ballheimer into the roof of the net.
Manager Fisher made a double change bringing on Joe Price in midfield and central defender Jake Hoy, but shortly afterwards, Hoy was outmuscled by the giant Simpson chasing a through ball and the Harefield forward lifted his shot over Croft to reduce the deficit.
WGC were straight back on the offensive and Yasir Rashid had an effort blocked on the goalline by defender Murphy, while a long range chipped effort from full-back Sam Perrin sailed just wide of the far post.
Seven minutes from time, Parkinson completed his hat-trick, Kersey made a strong run along the by-line and pulled it back for the winger to crash home his third of the game, for 5-1.
Two minutes later, poor Harefield marking at a corner allowed the kick to reach Yasin Boodhoo in the centre of the area and he hooked in number six.
WGC still weren't finished and on the stroke of full time, they scored an outstanding final goal.
Youth teamer Phil Ball, on for the last 13 minutes for Welsh, delivered a long cross that Robins retrieved with a spectacular overhead kick back into the area, Rashid striking an instant drive that billowed the netting, for a 7-1 final scoreline.
WGC: Croft, Perrin, Boodhoo, Mensah (Hoy 60), Welsh (Ball 77), Payne, Dimmock (Price 58), Robins, Rashid, Kersey, Parkinson.