Try
WWW.un.org
and look under employment opportunities / Logistics
You can fill an online CV which they look at and try to fit you up with the appropriate job, although have not seen ramp tramp jobs as such.
They do employ indigs for work but mostly on airfields work from what I understand.
They do 6 month contracts and can mess you around as in my last application was given 3 days notice to move!!
VEEPS