Be aware that in most (all?) Councils it's a requirement for the land to be zoned for multi-unit dwelling in order for it to be legal to rent out the house and granny flat separately. It would be illegal to do so in most standard low-density residential neighbourhoods.
Separately from the issue as to whether you're allowed to rent out the two buildings separately, you'd have to ensure that the granny flat is up to building code as a "habitable dwelling". There are various requirements on the building in order for it to be a habitable dwelling, such as amenities provided. Even if a tenant's happy to not have a kitchen, for example, and only eat take-aways, if the building's not classified as a "habitable dwelling", it's illegal to rent it out.
I'm sure there are people who do it illegally (not josko I'm sure

), but you need to be aware that doing so would probably negate both your home and landlord's insurance.
I know of what I speak, because I had a "studio"-type apartment at the rear of my student accommodation which I spent at least $10K renovating a few years ago. I'd naively assumed that because it had been rented out previously, that it was "kosher". As I was finishing, the neighbours complained and the Council visited and told me I couldn't rent out the room. That was bad enough news, but what
really burned me is that the reason the neighbours knew it was illegal was because they had owned the property previously, and they were the ones who illegally converted the shed in the first place, and they'd rented it out illegally for years.
(The same neighbours complained that one of my windows was illegal - because they'd put it in without permission! - and it cost me $15K in architects fees, plans, and applications to get it made legal following their complaint.

Then they wonder why I'm not sympathetic when they ring me at 3am and tell me that my tenants are making noise!

)