These are a few of the locations that I've tried sandboarding
Pebbley Beach (AKA The big dune around from soldiers) 
Pebbley Beach is on the Central Coast, NSW, Australia and is north of Soldiers Beach and south of Norah Head.
Pebbley beach pretty much just consists of one very steep, very high sand dune. This is a popular sandboarding spot, mostly with people
who have homemade sandboards, bodyboards, for sales signs etc.
If you're looking to do some tobogganing then this is the perfect dune, Only problem is that the sand is soft, so it's a hard walk back up, once you've made it to the bottom. There also isn't a whole heap of variety if you're looking at doing standup sandboaring.
North Entrance (AKA Tuggerah Beach, The Entrance North) 
North Entrance is on the Central Coast, NSW, Australia and is North of The Entrance (across the bridge).
This is the beach that I go to the most and also my favourite. The dunes are all fairly small but have a good variety in them, so there are the real steep ones, the nice curvy ones and of course
plenty of jumps. Also being small it means you don't have to walk to far to get back up to the top of the dune.
The dunes are broken into two sections, there are the southern dunes which are north of the surf club and south of the houses. Or there are the northern dunes, which are north of the houses. I prefer the dunes north of the houses as you can get some real good variety up that end, and the beach is fairly deserted which means aside from the fisherman down by the water you generally have the whole place to yourself.
Stockton Beach 
Stockton Beach is North of Newcastle, NSW, Australia
I've been up the northern end of this beach and it's awesome, it's just km's of massive sand dunes.
I was boarding on some dunes that you could walk to from the beach and they were great. Then
a group of us paid one of the asian tour buses to go up into the big dunes (hard to get to without a 4wd, not impossible just hard)
and we spent the next few hours sandboarding some of the biggest dunes I've ever been on, it was great.