The former prime minister and foreign minister says the death toll in the country - now estimated to exceed 60,000 - is obscene.
"It is like watching Rwanda," Mr Rudd says in an opinion piece published in the West Australian newspaper on Wednesday.
Mr Rudd earlier this week called on the world to consider arming Syria's rebels, who have been fighting to oust Assad since March 2011.
He now says the international community must also start mapping out ways to avoid a post-conflict disaster if Assad does fall.
"Prospects for post-conflict political and sectarian violence at this stage are high," he said.
"Again, this means that planning for an effective international intervention if and when the regime collapses is now critical."
Preparations need to be made for a properly constituted and trained UN force that could be rapidly deployed on the ground, he says.