Just curious (I'm WA so don't know the areas you speak of), have you tried towing a standard trailer on the beach there?
If not, beg/borrow/steal a 4x6ft trailer and try that first. See how it goes, if it does fine load up a little weight in it and try it again.
If it works with a 4x6 and your new trailer follows the same wheel track you'll be miles ahead.
Depending on how boggy it is you may need to do nothing more than let the tyres down to an appropriate pressure!!
If you have to hit the highway for many k's to get there, towing that weight might be ok or might be somewhat arduous..
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