I've recently launched a community-minded website where Charities can post their technical needs (i.e. new website or programming needed) and newcomers to the tech industry can carry out the work in exchange for the experience, reference/referral etc. Its a win-win for australian developers. Please support the website by registering and perhaps it will open your network to paying clients down the track.
It's probably not a good look to start off a potentially beneficial project by spamming forums... next time you think about joining a forum/group simply to promote your venture, you might want to think about reaching out to the forum admins/managers first?
For others looking to work with non-profits in order to build up a portfolio, or indeed just from the goodness of your heart, another site to also check out: www.goodcompany.com.au