Wilbunga (might be Wilbinga) is easy ... head north from Wanneroo Road toward Lancelin. It takes about 45 mins to get to the turnoff... it's signposted ... VERY big sign Wilbunga Grv or something like that. It's a picnic area ... stop here and let the tyres down to 16-20psi. You'll see a track heading west ... there are signs on the trees saying "4wd Recovery Service" .
Bascially, follow the trail (it's a big track ... just sandy and a bit rocky in places) as far as you can west and then you come to a t intersection. It's a 'T' becuase there's a bloody big sand dune infront of you. Go either left or right and then follow the tracks over the dunes. That gets you to the ocean. Just be careful at this time of year as the beach is wet and boggy. I found out the hard way in May this year ...
... so long as the driver is ok driving on sand and you have air compressors it's a great overnighter if you like fishing. It's appox. 15k's from Guilderton ... just depends on how far north you head from the 'T' .
Hope this helps.
If you have a gps I can post the waypoint to the Wilbunga turnoff but as I said you can't miss it.
EDIT: Just checked the map it is Wilbinga Grove and it's approx. 40k's from the Burns Beach turnoff.