A director of a duck tour company in New Zealand said the deaths of 17 people in a duck boat accident in the US was tragic, but the boat should not have been operating.

Seventeen people, including children, died after a boat carrying tourists on Table Rock Lake, in Branson, Missouri, capsized and sank in stormy weather on Thursday night.

The incident is being reported as the deadliest such accident in almost two decades, after a similar sinking in Arkansas killed 13 people in 1999.

In their initial assessment, authorities blamed thunderstorms and winds that approached hurricane strength, but a full investigation is under way.

Trevor Weir, director of Rotorua Duck Tours, said he had been following the coverage of the accident and was disappointed to see the conditions in which the boat was operating.

"Seeing some of the footage of where they were operating is just ridiculous. I just don't know why they would be operating in conditions like that. It is absolutely crazy," he said.

Rotorua Duck Tours has been operating for 15 years, and Weir said in its 300,000 trips, it has never had an on water incident in the duck boat.

"The lakes we operate on are very calm and we are regularly audited by Maritime NZ," he said.

"We are only allowed to operate on a lake with a wave height of under 40cm, if it goes over 40cm then we will pull the pin on it.

"Seeing those guys operating in those conditions was very disappointing. It is just like anything, if you are operating something outside of its operating parameters, then you are going to have issues."

Weir said NZ duck tours operate under very strict conditions, and in very different terrain.

"All the lakes we operate on are crater lakes, so they are very sheltered.

"[The US tour] was operating on quite an open lake with big waves, and it has obviously come up quick on them.

"We just don't get those conditions. You can get conditions similar to that on Lake Rotorua but we don't operate on Lake Rotorua," he said.

Weir said when operated under the right conditions, duck boats are very safe.

"I have absolutely no problems with our safety systems and everything we have in place to guarantee safety in the water," he said.

"Our operation is safe and as long as equipment is being operated within its parameters, which we well and truly are, then we guarantee safety."

Suzanne Smagala with Ripley Entertainment, which owns Ride the Ducks in Branson, said the company was assisting authorities.

She said this was the company's only accident in more than 40 years of operation.

The boat was carrying 29 passengers and two crew members on a pleasure cruise, and everyone aboard had been accounted for by midday Friday.

Seven of the 14 survivors were hurt when the vessel went down. At least two were hospitalised in critical condition. The captain survived, authorities said.

Since the accident, a private inspector has come forward and said he warned the company about design flaws putting the watercraft at greater risk of sinking.

Steve Paul, owner of the Test Drive Technologies inspection service in the St Louis area, said he issued a written report for the company in August 2017.

It explained why the boats' engines — and pumps that remove water from their hulls — might fail in inclement weather.

He also told The Associated Press that the tourist boats' canopies make them hard to escape when they sink.

Duck boats were originally designed for the military, specifically to transport troops and supplies in World War II. They were later modified for use as sightseeing vehicles.