My checking (with expertflyer) shows T9 on the outbound (UA840) on Nov30, but return UA839 is T0 on Dec20. Consequently the fare class that United.com uses is W on outbound and Q on return.

Another invaluable resource to check is http://www.itasoftware.com Click on "looking for airfares" in top right. Sign in as guest if necessary, enter details (including sales city of MEL or SYD if you want results in AUD) and search. This will show you hundreds of options over many airlines. When you complete your selection, be sure to click on "show booking details" to get fare basis.

itasoftware is very powerful and can show fares that are published and those that are available. (select or deselect "check for availability" under more options"). Itasoftware confirms that TLAU422 is the cheepest theoretical fare (AUD 617 each way, plus taxes and an outrageous fuel surcharge, total AUD 1677 round trip). But as I've noted above, T is not available on the return dates, so you are forced into a higher fare basis... over AUD2100.

itasoftware suggests that Air New Zealand is your best bet if shopping on price... AUD 1735.

Good luck.
DAK