The flight west crosses the date line and deletes a whole calendar day. Jet lag takes most of the next one. Fourteen days away is about four bases, not seven.
Google gives driving time with no stops. On winding roads it runs about 17% under New Zealand's own official figures. Eight of the 36 main routes will not fit in a sensible driving day.
Queenstown gets 15h35 of daylight in December and 8h47 in June. The Queenstown–Milford day trip needs 142% of a June day. It cannot be done.
Great Walks open on one fixed date each year. Miss it and you wait twelve months. Campervans go 6–12 months out for summer.
Written from New Zealand. Every figure comes from NZTA, the Department of Conservation, NIWA, Tourism New Zealand or Immigration New Zealand — and is checked against the others. Nothing is recycled from other blogs. The document is dated and versioned, because prices and booking dates change.
A couple spends US$12,000–18,000 on this trip. Across 11.5 usable days, one wasted day costs US$1,000–1,500.