I tried removing the version of eway that I was using (from the top of the list at: http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=742) and replaced it with the eway module recommended by the eway website: http://www.zen-cart.com/archived_contributions/payment/eway_xml.zip
...and now it seems to be working fine in live mode