The Secret of Brerreton Gully — Jessi Hammond

In the 1950s in rural Australia, Tom and his brother and their friends find what they think is an abandoned caravan on the Brerretons' land.
Except it's not abandoned.
And the man who owns it is not happy that he's been seen doing what he's doing.
He's so unhappy, in fact, that Tom is terrified that he and Fred and Bill will be shot before Tilly can get back with help…