Andrew Blackford of Queenstown asks :-
Why is it, that in the middle of summer there is such a distinct difference of the hour of sunset between say Auckland and Queenstown (upwards of an hour), but only a very small difference at sunrise (matter of minutes)?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
Around the December solstice, the beginning of the summer in most conventions, the sun rises in the southeast. So the sunrise line, the boundary between night and day, is tilted from northeast to southwest. It is roughly parallel to the east coast of the South Island and the Wairarapa-to-East Cape Coast of the North Island. The advancing sunrise line reaches Auckland and Queeenstown around the same time. (Of course its really us advancing to the sunrise line but it's easier to visualize the line approaching us.)
The summer sunrise line hits the North Island coast well before the South Island coast. That's why East Cape's Mt Hikurangi sees the sun before any South Island mountain. Whakatane, Wellington, Christchurch and Dunedin all get the sunrise within the same minute or two.
The 'summer' sun sets in the southwest. So the sunset line is tilted roughly from northwest to southeast, parallel to a line from Cape Reinga to Auckland. The rest of New Zealand tilts the other way: northeast to southwest. So the summer sunset line moves southwest-ward down the country: East Cape first; Cape Reinga to Napier next, then places south. Thus sunset in Queenstown is around 50 minutes later than it is in Auckland.
At the winter solstice the sun rises in the northeast and sets in the northwest. So the sunrise line moves down the country similarly to the summer sunset. Queenstown's sunrise is 45 minutes later than Auckland's. The 'winter' sunset crosses NZ like the summer sunrise. So sunset is around the same time in both Auckland and Queenstown.
In all this the sunrise and sunset times are taken as the times when the sun would appear or disappear on a flat (sea) horizon. Mountains will alter the time when the sun is actually visible.