depends on the parents they were bred from
ie: were they siblings, and then their grandparents
it happens on small islands over many years
if you can swap with someone else would be better
but then with the small gene pool you may be dealing with you may be breeding with the same genes
you may have no option but to let your torts bang shells
it would be good to sort out a register of tortoises in nz
and where they have come from to setup a breeding register