Director Peter Jackson shot the film on weekends over a four-year period with friends playing the lead roles. Jackson funded most of the film himself until towards the end of the shoot when the New Zealand Film Commission gave him money to finish his project after being impressed with what he'd already produced. There was never a script for the movie; each scene was filmed from ideas the director had come up with during the week.
Pete O'Herne hadn't shaved before his first scene. Director Peter Jackson told him to keep that look for continuity. So for four years, O'Herne couldn't shave for more than once a week and this habit was hard to break after shooting was completed.
While the original VHS box artwork had the alien flipping the middle finger, tapes distributed to US video rental stores were provided with an extra finger that could be glued in place to make it look like it was making a "peace" sign instead.
During a scene where Derek's van swerves off the road and runs over one of the aliens, Ken Hammon sat on the hood of a van because they weren't able to buy a camera mount for the hood. Peter Jackson was driving, but neither noticed a tree stump hidden in the tall grass. The van stopped immediately from high speed, launching Ken and the camera into the air. Amazingly, both Hammon and the camera came out fine, but said, "We ain't doin' that one again!"
Craig Smith was part of the original cast and was married and then divorced within the four-year time frame it took to make the film. Because most of the filming took place on weekends, he found himself written out because his new wife - a devout Christian - objected to him working on Sundays. Smith was written back into the film when he got divorced.
There are no female characters (although the voice of a female emergency-call operator is heard at the beginning of the film). Some of the aliens were played by women, but the aliens appear to be all males.
The "firearms" in the film are all non-functional replicas made by Peter Jackson. For example, what appears to be a WWII Sterling submachine gun is actually a length of aluminum pipe, a handle made from Fimo, and a piece of wood to stand in for the ammunition magazine. The actors shook the props to simulate recoil, and the muzzle flashes were added in post-production.
The banning of the film in the Australian state of Queensland in 1990 is regarded as its main reason why the Queensland Film Board of Review was disbanded, workers of the QFBOR regarded it as too violent, even altough it was already a censored version and the film had been approved by the New Zealand Film Commission, the nationwide Australian OFLC saw the workers as unprofessional for banning a approved film that was already censored, the OFLC then disbanded them.
The shot of Robert vomiting in the bowl is not Peter Jackson, but a hand puppet of Robert. Peter cast his face in plaster and it was made into a hand puppet. There was a funnel at the back of the puppet's head where the vomit was poured into.
All the dialogue in the film was dubbed in post-production. This was for two reasons: A) Part of the footage had no sound with it, since it had been filmed on Peter Jackson's own 16mm camera which didn't support sound recording, and B) Once the New Zealand Film Commission funded the remainder of the film, Jackson hired a sound camera. However, neither Jackson nor his crew were very skilled with sound recording and most of the dialogue was unusable.
The film started life as a ten-minute short called "Roast of the Day", which started shooting in 1981. The story concerned Giles (Craig Smith), an aid worker who comes to the small coastal town of Kaihoro on collection day. The young man encounters a psychopath named Robert who then pursues him. Giles escapes the madman and reaches an old heritage estate where he tries to contact the authorities. However, the same clan of cannibalistic psychos that Robert is from occupies the house and captures Giles. In an ironic twist the aid worker is later cooked up to relieve the famine of the cannibals. Along the way Peter Jackson added the "special forces" team that would be sent to rescue Giles, but in this early version the "boys" would turn out to be cannibals as well (they staged the whole thing because they like to play with their food). When it came time for Jackson to finally edit the film together he found that he had nearly 50 minutes' worth of usable footage. So with its length and being inspired by the 16mm efforts of Sam Raimi's The Evil Dead (1981), Jackson continued shooting to make the movie a full-length feature.
The name of the town 'Kaihoro' under attack is a Maori word that can be translated as either "Food Town" or "Fast Food" - Kai meaning Food, and Horo meaning Village and also Quickly, depending on the context.
'Mike Minnett' (Frank) was almost hit by a sledgehammer which flew out of an actor's hands during the filming of the cliff fight. It narrowly missed his face, sailing over the cliff and landing on the rocks below.