Indeed- my wee caravan's duratorques slowly sagged over the years (until they rapidly sagged halfway up Takaka hill, which wasn't fun.)
Light trailers (which means caravans) don't have any requirements for certification when modified or repaired; at least, not as far as a WOF is concerned (https://vehicleinspection.nzta.govt.nz/ ... l-trailers
). They just have to be strong enough for purpose. Your insurance company might take a different view.
Obviously, though, you want your axle to be attached to the chassis securely, in a way that won't unduly stress the chassis or axle. Solid steel spacers and high-tensile bolts is probably the way to go, but it's a good idea to have an engineer take a look at it- the extra distance between the axle and chassis might transfer a twisting force to the chassis rails, which might also need strengthening.