I am seeking a standard form scriptwriting contract applicable to the
laws of Australia eg the contract must have originated from an
Australian source. It will be to hire a writer to produce a script
from an already existing storyline. It will not be an option to
purchase the writers existing script.

I'll leave it to a researcher to source a free online contract for you
but this site can sell you one but I think you have to become a
member.
http://www.awg.com.au/artman/publish/article_46.shtml

Yes, I have already contacted these guys. You can only access their
agreements if you are an producer with credits in Australia, and
then,as you say you have to purchase and/or become a member.

Hi again. Just revisited your question and found that you'd not had
any luck with your contract. Here are a couple of sites that might be
useful either by having a thorough look yourself or by contacting them
directly and asking the question.
http://www.afc.gov.au/
http://www.if.com.au/
and 
http://www.alliance.org.au/weblinks/Itemid,23/
This last site is the Media, Entertainment and Arts Alliance which is
the union here in Australia. Their freelance department might be able
to help (although, I've rarely had any useful advice from the MEAA. I
think they're all actors 'between gigs' and get easily bored by pesky
members with queries. They certainly don't trouble themselves with
responding to emails so I suggest a phonecall and insistence on some
definite answers is the key here. I sometimes wonder what I'm paying
for! BUT, that's just my personal experience with 'em and don't let it
cloud your judgement.)

cheers

sm