YES, you, as an Australian, do have to pay again if you entered over a year ago. Unlike the fee for US and Canadian passport users (10 years) it is only for one year, as that is what Australia gives to accepted Argentinean applicants for visas to Oz.
Here is what your embassy in BA says, at http://www.argentina.embassy.gov.au/baircastellano/TAreciprocityTAreciprocity.html :
" If you are visiting Argentina you need to know the following information about payment of the reciprocity fee!
Australian tourists are charged a “reciprocity fee” of USD100. The fee is valid for multiple re-entries within a period of one year as of the date of first entry. The fee must be paid prior to arrival at all ports of entry, including airports, ports and land border crossings.
The reciprocity fee is payable on the Argentine Migration website. Travellers should print the receipt and submit it to Argentine immigration authorities at the port of entry.
Travellers who have not paid the reciprocity fee prior to arrival at an Argentine port of entry may be sent back to their port of embarkation.
For the full text of the travel advice, please refer to: smartraveller.gov.au "
I understand that the fee printouts are required at all border entries, though perhaps you may get lucky. I would not depend on it, as people have been sent back at even isolated border posts.
