The ship is being used by Interislander while the Aratere, which lost a propeller while crossing the Cook Strait in November, undergoes repairs.
Interislander general manager Thomas Davis today said nine crew members of the Stena Alegra were absent due to sickness.
They were from different parts of the staff, he said.
"While we strive at all times to keep to our timetables and minimise disruption to passengers and freight, strict international and local maritime agreements on minimum manning and muster levels apply in these situations to ensure the safety of our crew and passengers.''
Today's 3.30am Wellington to Picton freight sailing, and the ship's return 9.05am Picton to Wellington passenger sailing had been cancelled.
The same sailings for tomorrow had also been cancelled.