The 34-year-old, who plays for Irish province Connacht, was arrested by Welsh police following a match against English club Gloucester on Friday.
"Connacht Rugby can confirm that player Mils Muliaina is currently assisting the South Wales Police with enquiries relating to an alleged incident in Cardiff in March of this year," Connacht said in a statement.
"As this is a legal matter, Connacht Rugby will be making no further comment at this time."
Muliaina won 100 caps for the All Blacks between 2003 and 2011 before moving to Ireland last year.
He is due to join Italian club Zebre next season.
(Reporting by Ed Osmond, editing by Tony Jimenez)
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