Will this hard drive with an American power plug work in Australia with an power adaptor?
What do you mean by power adapter?
The rating sticker on the power supply should tell you if the adapter is a universal type, ie 100VAC - 240VAC, in which case all you would need would be a physical plug adapter. Otherwise, if the rating sticker specifies a fixed voltage, then you would need a 240V-110V transformer. Since such transformers are expensive, it would be cheaper to purchase an Australian adapter rated at 12V, 2A, centre pin positive.
Thanks, that answers my question.