Thanks everyone for the responses, huge help.

I think it's going to be easiest to just bite the bullet, ship home my bag and carry on the final three weeks of gear for my travels. Last thing I need is sorting out a lost/unclaimed bag in Sao Paulo while I'm wanting to just enjoy myself.

A new visa for Brazil (although good for 5 years, for future use) will still be significantly more than shipping a bag or box to Auckland from either BA or Santiago. If all goes to plan this time next year I'll have my NZ citizenship anyway so won't need a visa for Brazil.

Again, thanks for the advice!