Webjet.co.nz and House of Travel in my experience show the best comparisons.
I also query on Kayak to double check international rates, and then have a look at the individual websites of Qantas, Air New Zealand and JetStar.
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I’ve also recently built a flight search engine / deals site for Australia – Beat That Flight – where we post deals, and the prices search feature quite often beats deals sites like webjet/kayak, especially on trans-Tasman fares to New Zealand.
Sometimes it’s a lot cheaper to access the local web sites, for example, www.airnewzealand.co.nz instead of www.airnewzealand.com.
The usual Aussie website for comparing flight prices is Webjet, but in my experience they don’t list everything.
Also they charge a booking fee so use it to find a flight but you might get a better price on the same flight elsewhere once you have the details.
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