Scab is one of the most contagious of diseases sheep can suffer from and the disease arrived very early in New Zealand's history with flocks imported from Australia. It makes the poor sheep miserable, they lose weight, wool and condition.
The Provincial Councils issued fines, the sheep were branded and had to remain half a mile for the property boundary. Usually if one sheep was found with the mite, the whole flock was assumed to have it. The disease had an enormous impact on the profit of a sheep run.
John Acland and Charles Tripp were in a partnership running sheep on a pastoral lease in South Canterbury and the book 'Sheep Part 2' by P.G. Stevens has an interesting story:-
For a time in 1855-1856, Tripp and Acland ran the sheep they bought on terms from Moorhouse. Some years after that the Shepherds Bush sheep became scabby for a time, which the Mt Peel sheep never did. Sheep seldom crossed the Rangiatata unless driven over. Acland told me (Secretary of Agriculture), that very early one morning he saw a dog chase a sheep across the river on to the Mt Peel side. The sheep ran up the river-bed towards the gorge. Acland hurried back to the homestead and got several shepherds, and they rode up the run, rounding up every mob they met, to find the Shepherds Bush sheep and kill it before it could infect the Mt Peel sheep. After riding for several hours and looking through many hundred sheep, they found the straggler, caught and killed him and in half an hour he was cooking on the fire.
The author goes on to explain the movement of sheep in the South Island from the very early settlements, discusses the various breeds, their historical origins and their importation into New Zealand.