An investigation into the sinking of the HMNZS Manawanui will "get to the nub" of what happened when the ship sank, Defence Minister Judith Collins says, as she warns against a "rush to judgment" about last week's incident.

Collins recounted details about the incident in an interview with Q+A earlier today, where she also spoke on New Zealand's "very significant" credibility gap in defence capability.

The HMNZS Manawnui sank on a coral reef after it ran aground and caught on fire last Sunday. All of its 75 crew and passengers safely evacuated. It remains unclear why the vessel ran aground off the coast of the Samoan island of Upolu.

The Defence Minister praised a decision to evacuate the ship, which she again reiterated when asked if she had created the impression of being overly protective of the crew.

"If people had not been evacuated, when they were, we could have been bringing home some body bags," she told Q+A.

"What I said to the Prime Minister is, 'We could've had 75 body bags coming off that flight, and I am just so grateful nobody died. I am devastated by the loss of a ship, but I don't know that I could've coped with 75 body bags."

In her interview, Collins was also asked about the environmental impacts of the oil leaking from the HMNZS Manawnui and statements released by officials in Samoa.

She said the cause of the sinking may become known in a month's time, with the public release of an interim report into the incident.

"That will be what the inquiry looks at. We cannot rush to judgment," she said.

"We don't know who's at fault, what level of fault there is, and I expect the Court of Inquiry will find that and that will be released. Defence has been really clear back to me, as well, that they expect to release those findings with only redactions around some privacy issues, commercial sensitivity and national security, but they will get to the nub of it."

Collins said she wasn't aware of any engine issues the HMSNZ Manawanui could've had before the Samoa mission following speculation about possible causes.

Collins also spoke of her post-sinking conversations with Commander Yvonne Gray, who had previously been the captain of the $100 million ship.

The Defence Minister said she last spoke with Gray when she returned to New Zealand.

"I haven't spoken to her since I rang her on the Sunday afterwards. I said, 'I'm not ringing to find out what happened - because it'd be totally inappropriate'."

When asked how Gray responded, she said, "She just said, Yes, ma'am."

Later, when she saw the crew and passengers coming back on the flight, Collins said the people coming back on the flight were "pretty shattered". She said the ship's captain was putting on a "brave face" amid "enormous pressure".

"It looked like someone who was under an enormous amount of pressure and was trying very hard to have a brave face," Collins said.

The prominent breakdowns on the Air Force's Boeing 757s, used to transport the PM and others around the world, have drawn attention to the country's ageing defence kit.

The Defence Minister said she believed New Zealand's capability gap within its military was now "very significant", adding that defence staff were "experts at making old stuff move".

She said: "If I look at the kit, everything we get is old. Even the Manawanui, which I saw some commentators saying it was a new ship. Well, it was 15 years old when it was bought. It was never a warship. It didn't have the redundancies that a warship would have.

"The Boeings were bought when they were 10 years old - they're now 23 years old. We always seem to buy old, and we wonder why they break down. The Boeings have, to be frank, I don't want to jinx anything, you know, in this the last few months, they've been very active and they've been, fingers crossed, really good.

"Our people are experts at making old stuff move, but they shouldn't have to be within a changing world. Thirty-five years ago, the Navy had 19 vessels. We now have nine - actually, we have eight because one's at the bottom of the reef. That's the difference.

"We had, then, eight helicopters. Now we've got five. That tells you what's happened."

On changing geopolitics overseas, Collins added that she believed "the Indo-Pacific region is now a much different place than what it was, say, even five or 10 years ago."

Q+A with Jack Tame is made with the support of New Zealand On Air