Sorry probably a stupid question from a newbie... any help would be much appreciated.

If i txt a vf nz mobile, who has gone over to Australia (i am currently on supa prepay, with the add-on's txt4000 and best mate) does it cost me to txt the mobile that is in Australia? I have been txting this mobile, and have noticed that no credit has been deducted, hence why I just want to double check it's not costing the mobile that is in Australia for my txts.. (pretty sure it doesn't, but please enlighten me)

Thanks :)