Mountain biking in Hawks Nest

I don't know of anything in Hawks Nest but if you are keen for a bit of a drive Kiwarrak would be the go. It would take you an hour to get there from Hawks Nest heading north. It's listed to the right on the 'Trail Status' page. It has some great trails and is worth the drive if you can make it up there.

At Hawks Nest, there is a single track that winds its way north - it starts near the HN surf club, and goes for a few km. It's sandy in places. You then head west to the main HN-Mungo Brush Rd, and return to HN - or - try some of the 4WD trails west of the road.

There are also a few short lengths of single track in the bush east of the Jimmys Beach caravan park.

As well as all the above.
If you can drive from HN past Mungo Brush you can park at the start of the old Seal Rocks road. I haven't ridden this for a few years. The 1st 10km is pretty bumpy, but after the junction with a management trail (its obvious - on the left (9km) just short of the big gibber the road turns into a nice fast fireroad through forest, all the way to the tar Seal Rocks Road (20km). At 8km and again around the big gibber are a number of side access tracks (on the right going north) that will get you through to the surf beach. (you'll have to stash the bike and walk a few 100 ms over the sandhills.)
On the way back you can take the (now) right hand branch (going south) near the big gibber (I think the first few Kms, maybe 3, will be very sandy) and explore some side trails, to Shelly beach, Tickerabit (I think the camp site has been closed for a few years) and Johnson's hill - all are on the water (lake side) so good to cool off.
There are other remnant 4wd trails up there, but I don't know their status.
I guess you can do 20 to 50 odd Km there depending on your route.
Its all firetrail or management roads and be prepared for sand of course. And no hills.

As others have said its worth going up to Kiwarak for the day, some great singletrack trails.

http://maps.google.com.au/?ie=UTF8&ll=-32.485149...