The Kaiarahi was left with a large hole that spread across a meter in its hull after it smashed into a Wellington wharf when it came into berth last Sunday.
Interislander had expected the ferry to be out of action for a week.
"While we are tracking well with hull repairs, high winds and rain over the last couple of days have continued to delay the use of a crane and we are using temporary scaffolding to complete part of the repair work.
"We are working around the clock to get safely back into operation."
Interislander was working to contact all its affected customers who were expected to travel early this week and work out new travel time options for them.
KiwiRail's investigation into what led to the collision was ongoing.