Adding www to url results in Not Found error

Hi,
When creating my WP site I used http://nyac.org.au as I thought this wouldn’t matter and it would resolve either http://www.nyac.org.au or http://nyac.org.au but it doesn’t. It simply shows the servers default page instead when you hit www or on any page lower than that an error:Not Found
The requested URL /power-and-politics/rules-about-power/ was not found on this server.

I’ve tried going into Settings > General and updating it there but this causes the same error as above across the whole site. None of the admin pages are accessible and I’ve had to dive into the db and manually change the urls back.

Is this a WP thing or a server side/DNS thing?
Any help greatly appreciated!