TL;DR: Emus. Lot and lots of emus, eating farmers' grain.

Longer version:

After the First World War, Australia gave a lot of previously unfarmed land to veterans to farm. This meant that many farms were now in areas previously grazed by emus. To make things worse, there was a drought, reducing the grazing (do emus graze? I'm not a biologist) area of emus further. This meant that the huge birds were increasingly bothersome to the farmers, destroying their crops.

The farmers requested weapons (particularly Lewis machine-guns) from the government to kill the emus. Being veterans, they knew how to use them and imagined they would be very effective against massed emu herds. The government, perhaps not trusting the veterans (who were disgruntled over lack of promised subsidies and other government policies) with machine-guns, instead sent a detachment of soldiers.

Long story short, the soldiers were largely ineffective. The emus, when fired upon, scattered at high speeds, making it almost impossible to kill many at a time. The operation became a favorite target of the media, and even the military leaders made tongue-in-cheek observations about what a mighty foe the emu is. One famous quote from Major C. W. P: Meredith goes:

If we had a division with the bullet-carrying capacity of these birds it would face any army in the world. They can face machine-guns with the invulnerability of tanks. They are like Zulus whom even dum-dum bullets could not stop.

(From this newspaper source.)

Though a few emus were killed, a lot of ammunition and other resources were largely wasted. The emus continued to be a menace for decades, though farmers (without direct help from the government) killed hundreds of thousands.

Here is a good article, written for a general audience, on the subject.