Good luck sending 60kg to Aus ! I had some freak in Russia after a pair of minty black ones I have but the £400 freight quote put him off.
Looking at that picture, apart from the slight discolouration, what is the problem with your leather ? If it is not badly cracked, you are much better off getting it 'fed' and recoloured locally.
Any used covers you buy in the UK will have probably been 'Connolised ' already and the chance of them matching your other seat are zilch.
E34 interiors along with most other E34 parts are worth foook all here. I have two really nice undamaged E34 black leather sport complete interiors for sale at £275 a pop and all I am getting are council freaks annoying me and wanting to talk about their £200 bridge dodgers
What is a really nice black* leather sport trim for an E34 worth in Aus ? I have a frein in Perth who runs a BMW repair/ parts shop and I am trying to talk him into buying lots of frag (my) and shipping a container load out there.
*In the Uk , black interiors are far more sought after and hence more valuable than all other colours, this applies to all BMWs and most other makes.
Maybe with your climate and the excess of kiwis and other flotsam you have there

grey or beige / tan might be more sought after ?

and hence more valuable .