I've never seen it as an official Qantas policy (though, mind you, I've never specifically looked for it either). But I have been on two or three Qantas flights where it has been made perfectly clear that passengers would not be permitted to do this. Perhaps it's up to the individual pilot, I don't know.

I raise it only because it would seem a waste of money to buy three seats in a row so that you lie down only to discover that it wasn't allowed! That'd be a lot of space for your book and so on...