Seeing as you're not getting Addis specific replies, this is for Nairobi but the rules should be similar:
As you have an Aus passport the nice people at your High Commission give the letter of introduction for free.
I got a 2 month tourist visa in Nairobi in Jan '11 but it seemed random as to whether travellers were given 2 week transit visas or 2 month tourist visas. They didn't ask me if I was driving so I didn't tell them, so didn't give a copy of my carnet.
I was advised by a friend to get my visa in Nairobi as his experience in Addis was that the visa was more expensive and the embassy staff not so friendly. From the date of issue I had 2 months to enter Sudan.
As far as I know there isn't a 30 day tourist visa. No idea if a transit visa can be extended.
Last edited by misterpaul; 5 Dec 2011 at 09:33.
Reason: keep thinking of stuff to add