The Arrogant Horseman: Book 4 of the Van Diemen Chronicles

Through the manipulation of the secretive McKay Group a small religious cult has now become the ruling regime on the mainland of Australia. Their leader, the Governor-General Rezaul Dean, is intimidated by the growing influence of the Professor and now sends heavily armed soldiers to deal with the Tasmanians. Guns from the mainland against the bows and arrows of the Islanders. It should be a simple task.