Deckie PM me mate and I'll give you a call

I built a trailer for steep two years ago it's a ripper and has since that trip three times plus the holland and central desert road without issue BUT I built it too high and the suspension too tough in hindsight..... I need over a tonne it it before the springs even start working properly!.... You're welcome to come have a look if you want - I'm in mullaloo area 

hey Deckie , IMO.  The best place is the axle on top , unless you really require the extra height or clearance , the main issue would be on a rough track the axle / spring clamps gives way the axle although restricted will remain inside the spring , trailer area where as under slung axle when clamp gives way the axle can only go awal and cause major issues  

 Here is one I modified for a trip to Steep point and DHI 

first thing I decided was to go with 760kg springs ... After building one a few years back with 1200kg springs and hating how stiff it was..

45mm, 6 leaf ,eye to eye springs .. Then custom made some U-bolt brackets that incorporate a mount for ALKO caravan shocks 

Changed the 14" ford rims and hubs for some 6 stud land cruiser (Hilux) hubs and split rims with 235/85/16 tyres that are the same rolling diameter as the 265/75/16 that are on the HILUX..

towed like a dream ,shocks barely got warm even after hours on the corrugations on DHI !!

i also fitted a HILUX spare tyre winch under the trailer for the spare .. And changed the drawbar to have a gooseneck of sorts so it sits dead level behind the Hilux

Loaded right up with 8 blocks of Emu export / VB gas bottles 

110l esky FULL of block ice, 6 full Jerry cans, gas bottles, fishing gear and 2x HOBIE PRO ANGLER 12's

still had plenty of travel and flexed just nicely ,,

really happy with it 

having a prick of a time uploading pictures

Hi Luke, sounds like the one I am building lol.
But maybe I have gone a lil heavy on the springs but I wanted Galvinized ones & they start at 1100kg.
I thought about shockies but for one trip a year I found it not realy required. ( I will probably regret it ).
cheers & thanx for your input, it is appreciated.

 Sure, it's nice to not have them rust, but there are a lot of cases of them failing very prematurely. The heat applied when they are dipped stuffs up the temper apparently. Wouldn't use them, myself. I hope you are going multi-leaf, hung at both ends, and not those god-awful parabolic single leaf slipppers...

 Shock absorbers would make a big difference to the way a trailer rides, most people don't go to the trouble. The combination of the larger rolling diameter of the 16 " wheels and the shocks would eat the corrugations up. I've often toyed with the idea of fitting shocks to the boat trailer just for the annual Ningaloo trip. Just take them off for the rest of the year, as the shock shafts would no doubt rust and taske the seal out in no time. Anyone ever fitted shocks to a tandem boat trailer?

 Shocks definatley made a huge difference, and for only $109 delivered to my door (no brackets supplied) it was well worth the money.. i wanted to upgrade to a heavier axle and bearings than i had but was running out of time before the big trip.. 

Maybe on the next custom trailer build..

 Built my own boat trailer to put under the 600 Navigator , just to manage that Ningaloo station track, after the original failed. Only built it on an old roadmaster design, just made everything with heavier materials (4mm and some 6mm) with a proper load sharing tandem multi-leaved suspension to replace the parabolics. Could drive that bloody road at any speed I wanted. Now I have a fully rollered Roadmaster with the tilt frames, with a different boat,  lovely to launch/ retrieve with, but only 3mm material, just don't trust it on that road with a glass boat on it. So it's back to a very slow trip on the dirt bit.  I have a dinghy badly in need of a trailer, and I can't help myself, have to build something that will manage something like the Port Warrender track, not just buy one off the shelf.

I will be looking at the shockie option as well, I like the set up that luke has thanks.
Also will be taming it down to 850kg springs to soften it up a little, should lower the trailer by doing that.
Thanks for all your input folks, much appreciated.