WIK Consult, a German telecoms advisory firm, has studied the Trans-Tasman roaming market for the Australian Government which resulted in the following report published in May 2012:
http://www.med.govt.nz/sectors-indus...IK%20Study.pdf
Quite an interesting read as it explains many commercial and technical basics of cellular roaming by also making references to the experiences of European roaming regulation.
Further reports issued by the Australian Government:
http://ris.finance.gov.au/files/2013/02/03-TTMR-RIS.pdf
http://www.med.govt.nz/sectors-indus...t%20Report.pdf