The Diprotodon is referred to as a giant Wombat.  It is a reasonable description and there is evidence of a close relationship.

Actually there were several types of Diprotodon.  The biggest was Diprotodon optatum.  This grew to several tons in weight although its exact size is uncertain.  It was a similar size to a Rhinoceros although it did not have a horn.  Certainly the Diprotodon was smaller than an African Elephant although it may have had a short trunk.

Diprotodon optatum is the biggest Marsupial known to have existed.

Diet

There is little doubt that the Diprotodon was an herbivore.  There is strong evidence that it was both a grazer and a browser.  They had front feet that appear to be good for digging so they may have eaten roots at times.

Wombats are burrowing animals, but it is unlikely that an animal the size of a Diprotodon lived in borrows.

Intelligence

The Diprotodon had a very big head, but it appeared to contain air spaces and was not completely filled with brains. 

Perhaps the most intelligent of the current Marsupials is the Wombat.  It is not reasonable to assume that its giant relative was stupid.  We simply do not know.

Extinct

The Diprotodons are extinct.  Perhaps they became extinct about  25,000 years ago, but some experts put the date of extinction more recent than that.

Experts also argue about why it became extinct.  Humans came to Australia about 40,000 – 60,000 years ago so they co-existed with the Diprotodons for a long time.  There is only a small amount of evidence that Humans hunted Diprotodons, but based on Humans in other parts of the world, it appears to me to be certain that Humans did hunt the Diprotodons, but how much is very uncertain.

The theory that Humans simply hunted the Diprotodons to extinction is difficult to sustain.

Another theory is climate change.  The climate of Australia was getting drier at the time Diprotodons became extinct.   The trouble with this theory is that this group of animals had survived several other arid periods.  Also, although Australia as a whole was getting drier, it is a whole continent and not all parts were arid.

The third theory is that Humans with their use of fire were steadily changing the landscape.

My own feeling is that is was a combination of all these things.  Certainly the factor that was different from the earlier drying periods was the presence of Humans on the continent.

Bunyips

The Bunyip is a creature believed by some groups of Australians to exist.  It supposedly lives in water.  Although generally considered to be mythical, it is possible that Diprotodons gave rise to the Bunyip story.

Predators

We do not know what predators the Diprotodons had. It seems likely that the Marsupial Lion, Thylacoleo carnifex did hunt Diprotodons.  However, since this predator became extinct before the Diprotodon it is unlikely to be the cause of the Diprotodons’ extinction.

Sources

http://australianmuseum.net.au/Diprotodon-optatum

http://www.samuseum.sa.gov.au/lop/diprotodon.pdf

http://museumvictoria.com.au/melbournemuseum/discoverycentre/dinosaur-walk/meet-the-skeletons/diprotodon/

http://museumvictoria.com.au/prehistoric/mammals/diprotodontids.html

http://www.abc.net.au/science/ausbeasts/factfiles/diprotodon.htm

http://www.bbc.co.uk/nature/wildfacts/factfiles/3040.shtml

http://www.goyder.sa.gov.au/site/page.cfm?u=301

http://zoxaeus.com/ZTForums/index.php?showtopic=11722