For $149, Vodafone will sell you its USB Internet stick and 500MB of credit (though that's being upped to 2GB until June 30). Top-ups cost $19 for 500MB, $29 for 1GB or $49 for 3GB; each has a thirty-day expiry.
That's substantially pricier than Vodafone's prepaid options, which offer 5GB a month for $39.95. But if you don't foresee yourself often using the service, it might prove cheaper in the long run. Vodafone's recently upgraded coverage might also make it more tempting, though Telstra still outstrips it in that regard, and 3 offers a 12-month expiry option, albeit with much higher charges when you drop off the 3 network.