I'm a (very) big fan of the Zendesk team and story in general, and their offerings in this space should fit your requirements well if you've got the budget for their per-user licensing. You might also like to demo what the eXo Platform team have been up to as well. A few of their engineers orbit the WTD and Asciidoctor community as well.
Some of our beta users are using Corilla for their knowledge base. We're shipping an API soon to either connect with hosted KB suites or maintain your own. Obviously any open source project is free for life, so feel free to drop me a line.
And of course a shout-out to fellow Aussies, Atlassian. We stand by our good-natured ribbing from the old days that Confluence is "where good content goes to die", but I've really enjoyed seeing how the crew have built on what was a core expertise of ticketing systems. I'm assuming they still offer free licenses for open source projects so this could be an option. Like ZenDesk the company culture is amazing and their community teams are just wonderful to deal with.
Definitely check out Jive as well. To be candid I've suffered through two full product cycles of theirs in years gone by, but I assume the acquisition of companies like Producteev brought in the UX designers and product managers that they needed to focus on things like media and image management rather than adding Like buttons and gamification. I would be surprised if they hadn't hit a sweet spot by now given the depth of experience that company has, so another worthy contender.
Would love to hear what you settle on! Sounds like a great (and active!) community.