The ATO will consider whether you are undertaking a "profit making undertaking or plan". Under s 15-15 of the ITAA97 a profit making plan is viewed as statutory income and therefore assessable as ordinary income, not as a capital gain. This particular section of the income tax act doesn't rely on the need to be "carrying on a business" for such an event to be income. It is called an isolated transaction if your intent is just to make profits turning over your PPR quickly.

If it is purely your intent to make a profit selling a house you declare as your PPR then you are likely skating on thin ice. There is some fundamental case law underpinning this particular issue and you are unlikely to get away with it if the ATO decides to audit. Sounds like the accountant's advice is fundamentally correct but a good accountant would have also given you this background ruling advice as well.