The occult black magic ritual at the climax of the film was filmed on Friday the 13th.
The tunnels and gun pit scenes were filmed at Wrights Hill Fortress in Wellington, a real semi-restored WW2 bunker.
Paul Campion re-mortgaged his house in New Zealand to finance the film. It was originally just going to be made with his money until the New Zealand Film Commission backed the film and provided additional funding.
When Colonel Meyer interrogates Grogan and talks about 'New Zealanders, a bunch of farmers driving around the deserts of North Africa attacking by night and fleeing to hide like cowards' he is referring to the New Zealand section of the Long Range Desert Group, a special forces unit that was originally formed from the New Zealand Army stationed in North Africa.
The design of the German bunker is based on the real WW2 German bunker MP4, a Range Finding and Observation Tower which still exists in Guernsey, in the Channel Islands.
Jonathan King: the director of New Zealand horror film 'Black Sheep' plays a role as the suicide German slumped in the tunnel.