Are you aware of Tiger Air, Jetstar, and other low-cost airlines? Here: http://www.discountairfares.com/lcostaus.htm It might be cheaper to book your own one-way flights with these, and use your printed e-ticket receipt as "proof of onward travel" when entering each country.

The big airline search sites usually don't cover these new low-cost airlines. However, it wouldn't hurt to try your itinerary with these multi-stop searches:
http://matrix.itasoftware.com/cvg/dispatch/prego/multi
http://www.kayak.com/flights?mc=y
http://www.mobissimo.com/search_airfare_multi.php
http://www.airtreks.com
and http://www.statravel.com.au/cps/rde/xchg/au_division_web_live/

There are several ways to get the best airfare:
Try one of the big companies: Travelocity.com., Kayak.com or Expedia.com. The advantage to them is that they show many different airlines and the cheapest prices for both the specific date you want to fly and the dates surrounding that date. In case your plans are flexible. The disadvantage is that you are locked into the fare dates. If you want to change a date due to injury or an emergency, their fees can be extreme.
My personal method it to use Kayak as my initial check on fares and dates. I generally start at least a month in advance when looking for a ticket. The nice thing about kayak is that it hooks you up directly to the airline. Hence… you don’t get locked too deeply into one of the big three that charge up the wazoo for rescheduling. Then after I have figured one or two flights that are at the right times and right prices. In the upper left corner of the Kayak screen there is a feature that says “ track fares by email”. It’s referred to as Fare tracker and it really helps. Sign into this feature and it will update you daily on the changes in airfare. At least once in the month you are going to be able to find a fare that is usually a couple of hundred dollars less than what you have been quoted in the first place. A very cool feature.
Also, check out currentcodes.com and naughtycodes.com. these two sites have discount codes for some of the airlines and can save you some real money if you catch a special at the right time. They have discount codes for just about everything including airfares and Cruises. Not to mention coupon type codes for many stores and services.
If that is not what you want or you are in more of a hurry, then…
Try the airlines directly. Either their Web sites or direct telephone lines. Some of the Airlines offer low price guarantees meaning that if you find a cheaper fight on another Web site they may match the price or beat it. Each airline is different though, so you have to check them out individually.
Just remember, if you are going to try to get the cheapest flights, the airlines normally have restrictions. You may have to purchase the ticket 14 or 21 days in advance, there may be no refunds or changes allowed. You really need to know these restrictions.
If you try looking for a last-minute flight you will end up paying a lot more, although there are occasional exceptions. I personally, managed a last minute ticket to S.F on the Millennium New Years eve. And received a better price than my friends that had booked a month in advance! But… you cannot depend on this at all.
Usually travel agents do not get the lowest fares and sometimes they tack on a commission. Travel agencies used to be the best way to get the best airfare, but with the Internet, It is now just as easy for individuals to check themselves.
If you call an airline directly (they all have toll-free numbers), be sure to ask them if they have any specials. Often they do have specials, but you have to ask them about them, they won’t just tell you!
So be careful about restrictions on tickets. Be aware of charges for overweight baggage and remember that many of the very CHEAP fares you see in the Travel section of the local Papers are fares that are quoted, “without taxes” Taxes can really add up.
Have fun and happy travels.

Try http://www.zuji.com.au

They are pretty good…..or try the airlines individually.
Air New Zealand, Qantas, American.
Air New Zealand is usually the cheapest.

– Choose the same airline for all legs of your itinerary whenever possible so you can take advantage of all connecting and round-trip airfares.

– Try to fly on the least popular days: Tuesday, Wednesday or Saturday.

– Remember that Saturday night stays are usually required for most bargain airfares.

– Try to select the least popular flight times: mid-morning or late evening.

– Check with discount airlines for cheaper fares.

– Try to book as far in advance as possible. Generally try to purchase your tickets at least 21 days in advance of departure. Many airlines also have specials requiring three-, seven- or 14-day advance purchase.

– In general, the cheapest airfares are available in the spring and summer.

– If you travel around the holidays, book your departure and return flights a day or more before and after peak travel dates. If you're flexible, you might benefit by waiting until the last minute to book your flight because major carriers often run special sales. These sales, however, often have a long list of rules, restrictions and blackout dates.

–Consider booking tickets that come with restrictions because they also come with a cheaper price tag.

– Consider using alternate airports near your desired airport. The extra drive can save you hundreds of dollars.

–Be as flexible as possible with your dates.

If I can be of more assistance to you, please feel free to contact me and I'll do my best to help you out.