Labour Party leader Chris Hipkins says New Zealand First needs to back up serious allegations it made about the Aratere ferry grounding in a tweet.
He said if the party had any evidence to back up their statement it should be presented to the agencies investigating the accident.
The post on social media site X questioned whether crew negligence caused the ship to run aground soon after departing Picton last month.
"Is it true that the Aratere ran aground when someone put the autopilot on, went for a coffee, and then couldn't turn the autopilot off in time when that someone came back…?" the post read.
"If so, why haven't the public been told that?"
Hipkins told RNZ's Morning Report he had seen no evidence which could back up NZ First's assertion.
"I'm not necessarily sure if you're going to make that kind of allegation that that's the best way to do.
"Winston Peters is the deputy prime minister in the government, I think he should have a slightly higher standard for himself than that."
Interislander Executive General Manager Duncan Roy has said a regulated number of qualified people need to be in attendance on the ship's bridge at all times.
Roy said that includes the night of the grounding.
Merchant Service Guild vice president Ian McLeod said the party's speculation about what led to the ferry grounding was "incorrect and unhelpful".
McLeod said Maritime NZ and the Transport Accident Investigation Commission were interviewing crew members, canvassing the ship's logs and reviewing voice recorder files that ran during the incident.
He said he understood the Aratere's bridge was appropriately staffed and the crew had less than a minute to respond once they became aware of a problem.
"Because she hit the beach nose on [it] minimised the damage. If they'd tried to turn her away out to sea there is a chance of the stern and the rudders collecting the rocks as well - if they got that too late.
"These guys did exactly the right thing in the very short amount of time available."
McLeod said he doubted the guild's members were taking too much stock in NZ First disparaging their skills on social media.
"Our guys spend thousands of hours in training to get to the position of being in control of the vessel and one ill-thought-out comment by a politician isn't going to change the way we see our professional standing."
New Zealand First has declined to comment.
