Hamelin Hollow camping loop is part of Conto campground in Leeuwin-Naturaliste National Park.
This campground caters for small to large groups.   No dogs.   No Generators.

Location:
18km from Margaret River, 25km north of Augusta, 286km south of Perth

Directions:
Leaving Margaret River - travel along Bussell Hwy to Witchcliffe and turn right onto Redgate Rd.
Turn left onto Caves Rd and follow the signs to Lake Cave. 
Turn left before reaching the Cave at the campsite turn off.

snip from DEC website...
There are 22 camp sites at Hamelin Hollow.
Sites 5, 6, 7, 8, 9, 20, 21 are suitable for small vans.
All other sites are suitable only for tents.
Site 22 is suitable for disabled access.
...end snip

Facilities:
Picnic tables, fire rings and a camp kitchen with gas hotplates (no fridge).
Centrally located long drop toilets.
Some mill ends may be available for fire rings.  Fire restrictions apply November - April.
Water tap available behind camp kitchen. Must Be Boiled before drinking!

Costs:
DEC fees
Adults [16+] $7/night; Concession $5/night; Companion card free
Children [5-15] $2/night.  Children under 5 free.

Opinion:   
This 'group loop' can be extremely busy during school holidays.
This is great if you love mingling with others.
If you want a bit of peace and quite, ask the rangers about which of the other loops would be more suitable.
There is a lot to do in the "Cape To Cape" area, especially the caves and lighthouses.

http://tinyurl.com/7dghj77 - photos of the Cape to Cape area from DEC website