Wouldn't a LCC benefit more than a premium carrier from having the most fuel efficient aircraft as fuel makes up a higher proportion of cost? A332 will be returned to QF to replace B767.
Everything else being equal (rarely is, of course), an airline would use a more fuel-efficient aircraft in the role with
highest utilisation (hours per day). That way they get the biggest economic benefit (lower fuel costs).
Might or might not be the LCC subsidiary.