How long does it take to drive from Cairns to Perth, then up to Darwin?

And how much might petrol cost along the way?

Could this be done, comfortably, within the span of a month?

If you want to go to Perth from Cairns,I presume you want to go round the coast.If so here are some travel distances for you to chew over.First Cairns to Melbourne,which is heading south.2960 kilometres.That is not going along the coast,via Brisbane and Sydney which will add even more to trip.Then head west,Melbourne to Perth,3427 kilometres,then North to Darwin from Perth,just a quick trip,you best take lunch,4043 kilometres
Once you hit Darwin ,a short swim to Bali!
As for petrol,it will range from about at the moment from $1.30 to $2.00 Aust in the outback.
Lets say a medium 4cylinder car gets 10 kilometres to a litre,average fuel price,$1.50 multiply that all out and the distance is 10,500 kilometres and fuel costs $1600.
Remember this is not the short way and allow for your side trips.
Most Aussie would kike to travel this distance in 6 months taking it easy.You could do it in a week or so ,but what would be the point of it?
For distance in Aussie,go to www.whereis.com.au and click on get directions.Then click in the city and state where you want to go,hope that helps you out.Don't forget to visit the northern Beaches of Wollongong ,NSW

You could do that drive within a month but it certainly wouldn't be comfortable - it would mean driving about 10 hrs per day (not including breaks) on about half of the days of that month - obviously wouldn't leave you much time to see or do anything.
I think the petrol would cost you approximately $900-$1000 for a 4 cylinder car for the trip.

You would go Perth to Darwin then on to Cairns or Cairns to darwin then down to perth The trip perth to darwin is great you would be driving through The pilbara and kimberly regions It is 4,000 kilometres so you would want to call into a few places like Coral Bay and have a swim in the World heritage listed Ningaloo reef and also to Broome The pearling town of Australia It really is a super trip. I very much love darwin some great trips to cover here. You need to give yourself 2 weeks for this part of the journey. It would be an adventure of a life time. Fuel is dearer in the more remote areas of Australia I'm guessing you would pay at least $1.50 a litre for un leaded petrol possibly $1.60