Ferry operator Interislander says the Aratere will be brought back into port on Sunday while an investigation into the stranding continues.

The Interislander vessel lost its steering and ran aground near the port on Friday, before being successfully refloated on Saturday night.

Interislander executive general manager Duncan Roy said on Sunday morning divers had checked the hull and found it was still water-tight, and there appeared to be minimal damage to the vessel.

But an in-depth look at the state of the 26-year-old vessel was yet to be undertaken.

"The report from the people on the ship is they've tested the tanks, the divers have tested the hull and its operation."

Roy said the Transport Accident Investigation Commission and Maritime New Zealand will start investigating onboard the Aratere once it was returned to Picton.

KiwiRail chief executive Peter Reidy, appearing with Roy, thanked everyone involved in the refloating for their "significant effort".

Roy said it was a "very complex operation" and "humbling" to see how well the crew and experts did.

"Really, really impressive… one we were happy to resolve in 24 hours."

The Interislander team was currently working to find arrangements for passengers who had trips booked on the Aratere. One silver lining was the accident happened in a "low season" for bookings.

Some would be offered trips on the 29-year-old Kaitaki, or "alternative arrangements" would be found, Roy said, potentially by working with rival commercial ferry operator Bluebridge.

Another Interislander ferry, the 26-year-old Kaiarahi, would be back in service on 3 July. Only one ferry, Kaitaki, was operating in the meantime. Roy said Kaitaki had been operating with "100 percent reliability" of late.

Just days before the Aratere's accident, Transport Minister Simeon Brown raised concerns about KiwiRail's maintenance of its fleet at a meeting of the transport and infrastructure committee.

"We also want KiwiRail to make sure they're maintaining their existing boats to the appropriate standards, which has been a significant issue that we've been highly unimpressed with coming into government."

KiwiRail chief executive Peter Reidy did not want to comment directly on the minister's claims, but defended the Interislander's recent record.

"I'm incredibly proud - the last 18 months there's been a very big investment programme. You can see the ships in the dry docks, a big lift in capability. The last nine months we've had 99.6 percent reliability, on-time performance 92 percent - that is higher than global airlines. So we are very disappointed about what's happened in the last 24 hours.

"We'll do the work, we'll do the investigation, but we're very proud about what Duncan and the team have done to lift the performance of these ships… I'd take my own family on those ships right now."

Reidy said he had a "very frank" conversation with the transport minister on Saturday night, during which the minister praised the successful refloating of the Aratere.

Roy said the Aratere had just been upgraded with a new steering system, seven kilometres of cabling, and a "new automated approach".

"The original steering system was coming into obsolescence and there wasn't enough repair parts in the industry that we were happy with. So we made the decision to preemptively replace that with a new, state-of-the-art system which we brought in with international partners."

He said the "comprehensive investigation" into the cause of the accident would include examining that new steering system to see if it was at fault.

Asked if they had a potential timeframe for the Aratere to be back in service, Roy said "no", saying they would wait for the investigation and they they would "have the facts".

Late last year, the government canned the Inter-Island Resilient Connection (iReX) project - which included two brand-new ferries - due to its budget blowing out from $1.45 billion in 2021 to $2.6 billion in 2023.

The New Zealand Council of Trade Unions on Sunday said it was "beyond time" the fleet, each nearing three decades old, was replaced.

"Fifteen billion dollars of cargo is carried annually by the Intersialnder - it's in everyone's interest to make sure that the service is seaworthy," said president Richard Wagstaff.

"The decision to cancel the replacement ferries should be urgently reviewed in light of the developments overnight. The replacement ships from Korea could have been here from next year. "

He linked the accident to the recent falling of a pylon, which cut power off to Northland earlier this week.

"Yet investment by government in infrastructure will fall every year under the new Budget. The priorities of this government are all wrong."

Infrastructure Minister Chris Bishop recently said he wanted to see more public-private partnerships to fund new builds, rather than taxpayer and ratepayer money alone.

KiwiRail Board to be refreshed

KiwiRail Board Chair David McLean has advised his intention to retire from the KiwiRail Board and his early retirement has been accepted by Shareholding Ministers, Finance Minister Nicola Willis says.

“As an SOE, KiwiRail is governed by its Board. As Ministers, we exercise our influence through our appointments to the Board and the expectations we set for it.

“Unrelated to the Aratere incident, we can confirm that on Wednesday KiwiRail Board Chair David McLean advised us of his intention to retire early, effective from 31 July. His term was due to finish at the end of October.

“We will now conduct a search for his replacement and are considering additional changes to further refresh the KiwiRail Board,” Nicola Willis says.

State Owned Enterprises Minister Paul Goldsmith says the Government is focused on ensuring KiwiRail takes every action necessary to maintain the safety and reliability of its ferry operation.

“Earlier this year we appointed a Ministerial Advisory Group to advise us on options for replacing the ferries. The Group made a series of recommendations to us late last week which are being considered by Ministers before we take proposals to Cabinet,” Paul Goldsmith says.