Welcome to the forum, Ozzie.
The obvious first answer is "if you don't need the extended tray, go back to a normal tray or get a tub". Tubs can be found on eBay when people remove 'em to put trays on, often picked up by people trying to build really cool trailers.
The reasons for the obvious answer are several and serious. Whether or not you have a weight distribution hitch (WDH), putting the caravan on the ball adds 240kg to some point behind the rear axle, adding that weight to the rear suspension and bending the chassis and the hitch mount downwards. The WDH does not remove that weight, it only twists the chassis up at the rear so that the front goes back down. The weight is still on the rear.
Now, think of the distance between the rear axle and the normal position of the towball as a lever. Grab a longer lever - the downward force is multiplied. Your WDH will have to cater for that - I'd guess a Hayman Reese 750 would be the one you'd need, the 500 is not going to do enough.
Add to that, your chassis may not like that much torsion (due to the lever effect). It's something I'd have an engineer look at. Your 2.4T van (new) would cost what, $60K? Having your hitch break and tear free so that the front of your van hits the road and activates the breakaway system is going to be scary. It can be avoided.
I'd not extend the towbar, personally. I think the Navara already suffers from a bit of distance behind the rear axle to the tow point. You'll find that your rear suspension (which is already insufficient for more than about 150kg) is going to be way overloaded and even if it's upgraded, it's going to work a lot harder - again, the extra lever effect.
That said, you haven't indicated how much extension you're talking about. If it's just a little bit forget what I've written and go for it!
