Sheep inbreeding

Just as incest in humans leads to genetic defect risks, closely related sires and dams can have defective lambs. One defective gene is normally recessive, but if lambs inherit two copies of a defective gene that runs in the family, they may have serious abnormalities. Inbreeding problems can occur in a small flock that uses its own rams, or gets in rams from another flock which has similar family bloodlines. The maths are complicated, but in simple terms a sheep should not be bred to its parent or offspring, its sibling or half sibling, the sibling or half sibling of its parents or offspring, or the progeny of any of these siblings and half siblings. The closest relationship should be half-grandson to half-granddaughter.
New cleanskin breeds may require extra care to avoid inbreeding:
Only a small number of the original sheep from the cleanskin breed may have come to Australia, either by import when that was still straightforward, or by embryo transplant or semen more recently. The original sheep may or may not have been excellent, but in both cases they provided a small gene pool. No matter how many sheep have been bred from them, unless new genes have come in through cross-breeding, the family gene pool is still quite restricted.
Buying locally may bring additional relationships. For example the 'grandparent' of the ram you buy locally may have come from the same flock as your original ewes, and may be related to them. This makes the inbreeding maths even more complex.
Kinky tail - a genetic spinal defect

Even if you are lucky and have no serious genetic defects, evidence shows that inbred flocks become less and less productive, with lower fertility and lower growth rates.
Inbreeding issues can have an important impact on small farmers with new cleanskin breeds:
A small farmer probably needs to buy a new unrelated ram (or use artificial insemination of unrelated semen) every couple of years. The cost of a good ram is high if you have only a few ewes, and a local unrelated ram may be hard to find.
If you plan to swap rams locally with another small farm, check that its flock does not have similar family bloodlines, and check whether you have similar Ovine Brucellosis accreditation status.
You can inbreed so long as you know you may have defective lambs that die, and you intend to use the survivors for meat. It is irresponsible to breed from them again, or to sell them for breeding. And check that any stock you buy are not inbred themselves.
You may read about 'line breeding' in studs, but small farmers shouldn't try it. It means that closely related sheep have been bred, and the resulting defective lambs plus the parent that passed on the gene have been slaughtered. Over several generations the line breeder wipes out all sheep with defective genes, and is left with a bloodline with only good genes - unless they have made a mistake!