These are a handful of mountain biking links I have come across on the net:

• TrailFlixs – Watch videos of popular Sydney trails before riding them.
• Bicycle Tutor – Videos on common bike maintenance topics
• Sheldon Brown – The late Sheldon Brown was a well known bike personality.  A lot of great articles on bike repair and maintenance can be found on his website.
• Munda Biddi Trail – A great multi day MTB trail network developed by the WA Government
• The Fat Hippy – A good resource about Blue Mountain MTB trails.
• Mountain Bike Australia – Offical website