I have a pair of scrubbies, their around 12 months old and are in 1200mm/600mm/600mm enclosures.
When they are of an adult size, I will be building an aviary for each. That seems to be how everyone does it, yet that's in the warmer areas of Australia, as they are from north Queensland, I don't think an aviary in sydney would be good for them through them winter months, but that's just my opinion.
If you have the room, I'd opt for a aviary, if not, the cage size you have in mind sounds good. The depth sounds okay, just go a bit taller.
Just remember, if you have a 2 story house, or tight doorways, try keep the snake housed in the garage, or a well opened room, as if you ever need to move, that would be a struggle to move.