Well, thank you gentlemen, not only for the fast replies but also for kicking my brain into gear.

Didn't even think about axle width. Went out and had a good look and would definitely need a new axle to give tyre clearance if shifting the wall.

Now, if that was first gear, time to change gear. Who mentioned DIY... I'm pretty handy around the tools, got some mates who can weld and owe me favours for free work I have done for them including one whose old man owns a steel manufacturing factory, so maybe a few beers and a bottle of scotch might ensure getting genuine prices on materials and assistance. Probably should have asked him first but wasn't thinking along those lines.

2nd gear... a lot more research including problems associated with upgrading a registered trailer or even building one from scratch. VicRoads can be a right royal pain at times but other times go out of the way to help. Pot Luck. The good thing is I have time on my side. This doesn't need to happen immediately.

Also a special thank you to Dazzler. I have found your article - Quality Budget Priced Trailer - http://www.myswag.org/index.php?topic=36094.msg578367;topicseen#msg578367

has been done before
have a look at some of these for ideas.
https://www.caravancampingsales.com.au/items/details/0/SSE-AD-7087473/?Cr=0&gts=SSE-AD-7087473&gtsSaleId=SSE-AD-7087473&gtsViewType=showcase&rankingType=showcase
https://www.caravancampingsales.com.au/items/details/2008-adventure-campers-daintree-4-2m-14ft/SSE-AD-15480839/?Cr=5
might be easier to find one of these second hand

Find another trailer is probably the easiest way. Unless you are doing it all yourself as in widening the trailer it will cost you a lot more.

Dan.

Price reduced to $21,500, negotiable.

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I don't know what gravel/road/ rough road experience you have, but my comment is take it easy/ slowly as it will reduce the chance of doing any damage to your vehicle or trailer. We found plenty of "bat outahell" drivers who were a real menace to themselves and others coming the other way.
The "Tip" should be just part of the journey , not the holy grail as some of the drivers seem to behave.
If in doubt about needing to replace any suspension parts, do it before hand. If your shocks/struts and possibly some bushes are getting towards 200 000km or more, replace them before you go.
Allow at least two weeks to get from Cooktown to the Tip and back to enjoy it.
Allow time to go Thursday island for the day.
There is a Cape York tourist website.
For us it's a been there once, not bothering to go back experience, plenty of others go there numerous times though.
We prefer more western locations and southern beaches where you can swim without blasted crocs.
Mark

thanks for the feedback, mate...

gravel roads are ok... have experience with that...
understand that there other things to see, will be putting things on our list
shocks and bushes have just been replaced... a bit more work to be done on vehicle and trailer, before we go...
we are allowing about two weeks...
have been west and south... this is a bucket list 'once in a lifetime' trip...