Blenheim is the most populous town in the region of Marlborough, in the north east of the South Island of New Zealand. It has an estimated ur…Blenheim is the most populous town in the region of Marlborough, in the north east of the South Island of New Zealand. It has an estimated urban population of 30,600. The surrounding Marlborough wine region is well known as the centre of the New Zealand wine industry. It enjoys one of New Zealand's sunniest climates, with warm, relatively dry summers and cool, crisp winters.