The Boy from Andromeda.

On a holiday to Mt Tarawera, teenager Jenny finds an odd shard of metal. In this third episode of the kids sci-fi series she meets its owner: 'Drom' — a survivor of an alien mission to deactivate a planet-annihilating space gun (aka Tarawera itself). They find themselves under siege from a Predator-like 'Guardian' of the gun. If Drom and Jenny and local kids Tessa and Lloyd can't defeat the mechanoid, catastrophe is imminent! The South Pacific Pictures series found international sales and cult repute.