The Quest (1986)

Synopsis
In this Australian children's adventure story, the young inventor Cody Walpole (Henry Thomas) is forced to move to the outback with his father's best friend following the death of both parents. Upon arrival, he becomes intrigued by local lore of a "donkejin," or "bunyip," a legendary, dinosaur-like creature that is said to inhabit a defunct mine that lies nearby. Soon, his girlfriend and her younger, wisecracking sister are indeed nearly attacked by something that resembles the bunyip, while rafting in the local lake. Cody begins to suspect that the bunyip is a kind of Loch Ness monster that inhabits the body of water, and he is determined to prove it. A fisherman has died in the lake under mysterious circumstances and it inspires Cody to confront the monster head-on. ~ Eleanor Mannikka, All Movie Guide