Its sailings have been canned since early March due to a gearbox issue, adding to the list of Cook Strait ferry cancellations.
Interislander general manager Walter Rushbrook said the return of Kaitaki would provide more certainty for the school holidays.
He said the ferry had undergone sea trials, assurance checks and had been signed off by an independent third-party.
The fleet would be checked more regularly, Rushbrooke said, and all 500 people booked to travel from tomorrow could cross as planned.
Rushbrook said the Kaitaki repair was "complicated" and needed a particular type of metal that was manufactured and shipped over from Germany, as well as "specialist technical support" from the Netherlands.
"The gearbox failure was a surprise, given it was overhauled late last year in drydock.
"Our focus over the past few weeks has been on restoring Kaitaki to service and offering customers booked to sail on Interislander as much certainty as possible about their travel."
Rushbrook said the company knew the cancellations had not been easy for customers and it was sorry from the inconveniences caused.
"As we look to the future, we're working on a number of fronts to make sure our ferry service is modern, safe and reliable.
"This includes purchasing two new, larger ferries that will be built at Hyundai-Mipo Dockyard in South Korea. These hybrid electric/diesel vessels are due to arrive in 2025 and 2026."
Transport Minister Michael Wood blamed previous governments for the disruptions, telling RNZ they had failed to plan for the long-term.
The current regime was investing more than $430 million on two replacement Interislander ferries, he said, but the first one would not arrive until at least 2025.
