Potatoes aren't native to Australia so first and foremost the environment isn't suited sometimes.

Secondly capital plays a role. They're frozen then processes, and chips (i.e. french fries) are in higher demand so what surviving crops are turned into those (the ones that don't hit the supermarkets anyway). It wasnt so much an actual shortage as a variety shortage. There was a lack of pre packaged mashed potatoes, hash browns and seasoned wedges. The higher demand was more profitable but that didn't stop people from asking "it's made from the same plant how do have chips but not wedges/hash browns/etc"

Edit: the chip (fries) becoming harder to get was due to a coinciding local tourist event that tripled this town's population for a weekend