To get a test before a holiday, then testing a relative abroad and mailing it off from there.
Thanks 
Hi,
I assume you mean Australia? ( AU)
If you order an ancestry kit and get it sent to your address in the UK ( or whatever country outside Australia ) ,
and you then send it here to Australia - the pre paid bag ( to return the sample) , I would think will be for within the UK use and not Aus handled international post.
If purchasing, if it were me, I'd get the kit sent directly from Ancestry to the person out here in Aus , otherwise I would think the pre paid bag in the kit mailed to your address outside Aus is hopeless out here, as the pre paid bag wouldn't be paid for with Aus post to send international to *Ireland (* where our kits are posted to from here in Aus) .
Kind regards
Ps they are pretty quick at posting the kits out, if what you're worried about is the posting times of receiving a kit here in Aus
from Ancestry.
The kits come from within Australia and not from overseas ( by an Ancestry's suppliers within Aus) . It should take no more than two weeks for the person to receive the kit in Aus.
*I Think mine took a week if that ( *obviously this depends where the person is in Aus of course )