To make the whole trailer wider you need to start at the chassis so that means building a new trailer, you could reuse the sides and possibly the drawbar but everything else will need to be replaced.
 I prefer to use what i have if i can but in this instance it wont work for you, if you wanted just to top to be wider then that can be done but you want the rtt sitting in the bottom, 5ft shouldn't be wider unless you drive a mini or suzuki.

Mate, you can always "check out" Palm Creek, Gunshot etc, they aren't too far off the PDR. Nolan's is only a short run from Eliot/Twin Falls.

This time of year [just after the grading has been done] is the best.

If you are going to TI, don't bother with Horn Is., unless you have a connection with WW11 history.

Hem

thanks mate... this time of year in 2025 is our plan... hence trying to plan now and start booking...

Mate, you can always "check out" Palm Creek, Gunshot etc, they aren't too far off the PDR. Nolan's is only a short run from Eliot/Twin Falls.

This time of year [just after the grading has been done] is the best.

If you are going to TI, don't bother with Horn Is., unless you have a connection with WW11 history.

Hem

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