Acting Prime Minister Winston Peters has alleged a cover-up at KiwiRail over the grounding of the Aratere last month.

There were allegations from coalition government partner New Zealand First earlier this week the accident last month happened after "someone put the autopilot on, went for a coffee, and then couldn't turn the autopilot off in time when that someone came back".

Party leader Peters on Thursday morning appeared to distance himself from the claim, before doubling-down on it.

"But here's the point - at the moment we heard this story, it was clear as daylight that something dramatically had gone wrong, that shouldn't have gone wrong, and we find out it was on autopilot." Peters said the party had sources on the boat itself, which relayed the 'coffee break' claim.

"I am the acting prime minister, and the reality is that we are wanting to hear the truth, but we don't want to delay any longer. And in a way, you can say I'm asking KiwiRail [to] front up - right here right now."

Labour leader Chris Hipkins was unimpressed with New Zealand First posting the claim to social media, rather than sharing whatever information it had with investigators.