The Valentine is completing technical due diligence in England now, before setting sail to New Zealand, to join the fleet in mid-December.
It's initially being leased for 12 months - for freight only.
Interislander ferries have had a series of failures in recent months.
The Aratere had mechanical issues last week - and the Kaiarahi is out of action until at least March with gearbox problems.
KiwiRail Group Chief Executive Greg Miller says having another ferry will provide resilience across the Cook Strait.