A statement issued this afternoon by Ulster Rugby read: "Nick Williams is unavailable for selection for Friday night's match against the Newport Gwent Dragons due to an internal investigation into alleged unauthorised absence.
"Ulster Rugby will make no further comment on the matter at this time."
The Auckland-born Williams had a terrific 2012/13 campaign for Ulster, with his barnstorming performances earning him the IRUPA Players' Player of the Year and RaboDirect PRO12 Player of the Year awards.
He signed a contract extension with Ulster in October, with the province securing his services until the summer of 2016. This season he has made 10 appearances to date, despite being curtailed by a recurring calf injury.
Speaking about the internal investigation at a press briefing today, Ulster head coach Mark Anscombe said: "We all hope the process doesn't take long - for all parties concerned it's best that these things are dealt with immediately and then we can all move on."