Membership of the Royal Victorian Order is at the personal discretion of the Queen – she does not act on advice from the British or NZ PM for these awards. The GCVO is the top award in the order – they have MVO, LCO, CVO, KCVO and then GCVO. They are given for personal service to the Sovereign.
The only title more senior to GCVO, that is in the Queen’s discretion, is the KG – Knight of the Garter which is limited to 20 living people, and Sir Ed Hillary had. Sir Paul Reeves and Dame Cath Tizard are the only other NZers (I think) who are GCVO.
Anyway, hearty congratulations to Sir Don for his award.