I am a member of the Greens. The main reason for joining them is that they have principles. The Greens Charter  guides everything, policy must comply with the charter and candidates agree to uphold the policies which have been ratified if they get elected. There are no “non-core promises“.
The policies of the Greens are all positive. The major parties have some mean-spirited policies that aim to help Australians by making things worse for people in other countries. Unfortunately for them the world is very inter-connected at the moment, so it’s difficult to harm other countries without harming yourself in the process.
As you might expect the Greens are very positive towards the environment. Anyone who expects to live for more than 30 years (or who has children) should be very concerned about this. Currently even the most cautious estimates of the scope of the climate change problem by reputable scientists suggest that there will be serious problems in the next few decades. If you are young or have children then you should vote for the Greens (that covers most people).
Many religious groups have determined that God wants them to help the environment, for example the Australian Anglican Church General Synod 2007 resolved that “the Anglican Communion’s 5th mark of mission to safeguard the integrity of creation and sustain and renew the life of the earth; and recognises that human activity contributing to Climate change is one of the most pressing ethical issues of our time”. Anglicans and believers in other religions that have similar ideas should vote for the Greens.
The Greens have policies that are very positive towards minority groups. If you are not a straight-white-Christian then this is a strong reason for voting for the Greens. If you are a straight-white-Christian but have compassion for others (as advocated in the Bible) then again voting for the Greens is the right thing to do.
The Greens policies are humane towards people in unfortunate situations, including drug addicts, the mentally ill, and the unemployed. When considering who to vote for keep in mind that at some future time you or a close friend or relative may fall into one of those categories.
Finally the Howard government’s industrial relations legislation is very bad for the majority of workers. If the Greens get the balance of power in the Senate then they will be able to get such laws significantly changed or removed.
As an aside, I never liked Paul Keating (our last Prime Minister) until I read his op-ed piece in The Age about John Howard .
Also here is an interesting article on preferences and why the major parties want people to misunderstand the Australian electoral system . In summary if you vote for the Greens as your first preference and they don’t win, then your second preference gets counted, and if that party doesn’t win then the third preference counts, etc. So if you have the two major parties in last and second-last position then your second-last preference may make an impact on the result! Putting Greens in the #1 position does not “waste” your vote. Parties get government funding based on the number of #1 votes, vote for the Greens as your first preference and you effectively give them $2 for their next campaign. Finally when a party wins an election they immediately look at where the #1 votes went and often adjust their policies to please the people. Vote #1 for the Greens and if Labour wins they will have more of an incentive to adopt policies that are similar to those of the Greens.
-  http://www.vic.greens.org.au/about-the-greens/charter/english/
-  http://greens.org.au/Election
-  http://www.theage.com.au/articles/2007/11/21/1195321862029.html?page=fullpage
-  http://www.theage.com.au/news/opinion/your-voting-preferences-really-do-matter/2007/11/19/1195321692088.html?page=fullpage#contentSwap1