I've come to rely heavily on websites which advertise various flexible flight search options, like nearby alternate airports and flexible date options. For just these reasons, plus nearly always the lowest-price result, I've been recommending
kayak.com unreservedly. At least for international searches, there are serious shortcomings with both kayak, and my second-favorite site, orbitz.com. Yesterday, I did a flight search on kayak, and I've repeated it today, for which ticking the "include nearby airports" box results in the lowest-priced result being $900
more expensive. Surely including additional airports shouldn't omit the original search! Similar things happen, unfortunately, with the flexible date option.
I suspect I understand the problem, which is that properly searching for flexible dates and/or airports is a very time consuming process, and your average user simply doesn't have the patience to wait for good results. So the sites don't bother with that, they just return mediocre results quickly. The point is easily illustrated on orbitz. They've got a very handy, in theory, feature: search for a flights which result in a trip of a flexible length, across a range of days. Perfect! Tragically, it simply doesn't return good results. When I searched for a 10-12 day trip from SEA-AKL, between 15 Jun 09 and 15 Jul 09, Orbitz flexible date search feature dutifully (and quickly) returned a lowest price of $1819. However, by doing a traditional search (i.e. departing on a fixed date and returning on a fixed date),
on orbitz, and repeat it across the dates which were part of the flexible search, you get much improved results, by about $250! The moral of the story is that if you're thinking of visiting NZ (or the UK, and you should be!), you'd look at the former, rather than the latter... Hopefully at some point there is a flight search site which can do flexible and accurate searches quickly, or perhaps just one which takes a while but emails you the results later?
dates 06/15/2009 to 06/26/2009, price $1,826
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dates 06/16/2009 to 06/28/2009, price $1,819
dates 06/16/2009 to 06/29/2009, price $1,586dates 06/17/2009 to 06/28/2009, price $1,819
dates 06/17/2009 to 06/29/2009, price $1,568dates 06/17/2009 to 06/30/2009, price $1,826
dates 06/18/2009 to 06/29/2009, price $1,599dates 06/18/2009 to 06/30/2009, price $1,826
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dates 06/20/2009 to 07/02/2009, price $1,826
dates 06/20/2009 to 07/03/2009, price $1,599dates 06/21/2009 to 07/02/2009, price $1,826
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dates 06/23/2009 to 07/04/2009, price $1,826
dates 06/23/2009 to 07/05/2009, price $1,617
dates 06/23/2009 to 07/06/2009, price $1,586
dates 06/24/2009 to 07/05/2009, price $1,599
dates 06/24/2009 to 07/06/2009, price $1,568dates 06/24/2009 to 07/07/2009, price $1,826
dates 06/25/2009 to 07/06/2009, price $1,599dates 06/25/2009 to 07/07/2009, price $1,826
[...]
dates 06/30/2009 to 07/11/2009, price $1,826
dates 07/02/2009 to 07/15/2009, price $1,827
dates 07/03/2009 to 07/14/2009, price $1,820
dates 07/03/2009 to 07/15/2009, price $1,820
dates 06/30/2009 to 07/12/2009, price $1,618dates 06/30/2009 to 07/13/2009, price $1,587
dates 07/01/2009 to 07/12/2009, price $1,600dates 07/01/2009 to 07/13/2009, price $1,569dates 07/01/2009 to 07/14/2009, price $1,827
dates 07/02/2009 to 07/13/2009, price $1,600dates 07/02/2009 to 07/14/2009, price $1,827
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dates 07/04/2009 to 07/15/2009, price $1,827