What does the NZ Thoroughbred brand DS stand for?

The DS could stand for anything. It could be a ranch brand, previous owner, many different things. The only way to know who his sire is, is to have his papers. Ususally horses are not branded for the name of the sire, or dam. So it is probably a brand to identify who he was previously owned by, or it is an affiliation with a place where he once was.