What is the meaning of the Australian nickname "Wacka" as in Wacka
Dawson, the character in the Alan Seymour play, The One Day Of The
Year?

Two things come to mind
In Australia nicknames are often derived from first names my added an
'a'
Mark -> Macka
Warren -> Wazza
But I can't work out what name Wacka would be derived from.

The WACA (Western Australia Cricket Association) pronouced Wacka is
the name of the cricket ground in Perth.

According to an online dictionary of Australian slang, "whacker" or
"whacka" means "idiot."

http://www.koalanet.com.au/australian-slang.html#W

I wonder whether "Wacka" may be a variant of "whacka."

Here in Australia we tend to call someone a "wacka" to indicate that
they are wacky, silly, idiotic. Not sure of its derivation but I'll
keep looking. In our vernacular, we call someone a wacka in the same
way that someone in the US might call someone a dufus or in the UK, a
twit. Not particuarly insulting but not exactly complimentary!

In the United States, the term "wacko" is often used to describe a
bizarre or crazy person. Such a person is said to be "wacked out."
There's also "wackaloon," which appears to be a portmanteau word
combining "wacky" with "looney."

I think "wacko" is used in the UK, too, since I've seen several
headlines in British tabloids that referred to Michael Jackson as
"Wacko Jacko."

I thought Mr Skelton would have provided the definitive answer on this
one but as another Australian I will have a go in his or any other
experts absence. The american "stressedmum" who wrote,"Not particuarly
insulting but not exactly complimentary" in regards a similar American
term to describe someone not being very smart or acting stupid had the
right answer for Wacka as well. In fact I think it is wacker rather
than wacka and I think it derives from the  commonly used word
"bushwacker", used years ago. I am not sure what that meant either but
suspect it was someone who lived in the bush,(australian
forests/mountains)and lived off the fruits of the open range also.

 John from melbourne

Wacka, rather than the proper given first name of Warwick, is used by
Australian men who are mates. Such infantilising of men's names is
common. eg Bazza (Barry), Macka (Mark) etc etc