Flights

I think this email explains it fairly well, with the important info in bold:

Dear Elin,

Thank you for your reply.

Air New Zealand do not have open-dated tickets. Availability of flights and fares is 11 months out from the date on which you are looking.

Please select your required outbound date, and the furthest return date possible. Immediately that you have received your electronic ticket, please telephone our Air New Zealand Contact Centre on 13 24 76 in Australia, and one of our consultants can add a note to your booking advising that we will allow one free of charge change to your return date. Any applicable fare difference would apply.

As soon as the return date you require becomes available, i.e. 11 months before return travel, please telephone Air New Zealand and change your booking. Generally, the earlier you change the ticket, the less likely there is to be any fare difference, or at least the fare difference should be minimal.

Once we have provided the free of charge change for your return flight, any further changes would incur the normal change fee, service fees, and any applicable fare difference, per person.

Any change to your outbound flight would incur all charges at any time, as it would only be the return flight that has one free change possible.

We hope this information is helpful. If you have any further questions you can reply to this email. Alternatively, you can telephone our Air New Zealand Contact Centre on one of the telephone numbers below, or +64 9 357 3000 worldwide.

Kind regards,

Andrew Air New Zealand Online Support

New Zealand: 0800 737 000 Australia: 13 24 76 USA: 1 800 262 1234 Canada: 800 663 5494 United Kingdom: 0800 028 4149 Overseas: +64 9 357 3000

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So given Andrew's suggested plan of action, I'm currently in the position where I must make the change to the booking as soon as it becomes available in September 2010 for August 2011.

Outbound Flight

From	To	Via	Date and Time	Landing At	Flight No	Terminal	Carrier
London - LHR	Auckland - AUK	Hong Kong	Thursday 12th August 2010 21:05pm	09:00am Sat 14th	NZ38	LHR Terminal 1	AirNewZealand

Notes on Flight

I booked it on Wednesday night, December 23rd, for a flight which is mostly on Friday the 13th. Because of this I got this return ticket for £850 and am well pleased ^_^ So in future- good time to buy is Wednesday night, and avoid times like January, after christmas, because that's when everyone will want to book (or at least, that was my logic).