A Japanese infectious diseases expert has made waves with videos slamming the government's handling of a quarantined cruise ship, racking up hundreds of thousands of views as passengers start to leave the boat.

The situation on the Diamond Princess was "completely chaotic" and violated quarantine procedures, said Kentaro Iwata, a professor at Kobe University, in unvarnished criticism rarely seen in Japanese officialdom or academia.

"The cruise ship was completely inadequate in terms of infection control," said Iwata in videos in English and Japanese posted late Tuesday.

Passengers have been allowed to walk on the deck of the virus-hit Diamond Princess if they wear face masks
Passengers have been allowed to walk on the deck of the virus-hit Diamond Princess if they wear face masks AFP / Behrouz MEHRI

He said he was so concerned at what he saw on the ship during a brief visit on Tuesday that he has placed himself in a 14-day quarantine to avoid infecting his family.

"There was no distinction between the green zone, which is free of infection, and the red zone, which is potentially contaminated by the virus," he added.

"I was in Africa dealing with the Ebola outbreak. I was in other countries dealing with the cholera outbreak. I was in China in 2003 to deal with SARS... I never had fear of getting infection myself," he said in the English video.

On Wednesday hundreds of passengers on the Diamond Princess who tested negative for the virus began leaving the ship
On Wednesday hundreds of passengers on the Diamond Princess who tested negative for the virus began leaving the ship AFP / CHARLY TRIBALLEAU

"But inside Diamond Princess, I was so scared... because there was no way to tell where the virus is."

More than 540 people on board have tested positive for the virus since the vessel arrived off Japan's coast on February 3.

On Wednesday hundreds of passengers on the Diamond Princess who tested negative for the virus began leaving the ship
On Wednesday hundreds of passengers on the Diamond Princess who tested negative for the virus began leaving the ship AFPTV / Quentin TYBERGHIEN

The ship was placed into quarantine two days later, with passengers confined to cabins except for brief trips on deck wearing masks and gloves, when they were told to keep their distance from other passengers.

Japanese officials say infections on board mostly predate the quarantine, which they insist has been effective. And on Wednesday, hundreds of passengers who tested negative began leaving the ship.

Government officials rejected Iwata's criticisms.

"Since February 5, we have taken thorough measures to prevent the spread of infection, including wearing masks, washing hands and using hand sanitiser," government spokesman Yoshihide Suga told reporters on Wednesday.

"What is true is that we are doing our best to take action to prevent infections," he said.

Japan's Health Minister Katsunobu Kato also defended the government's approach.

"If symptoms develop, of course not only that person but also those who had close contact with the person are told to refrain from serving, and workers follow rules such as wearing masks and putting on gloves," he added.

"According to experts, the infection is under control."

Japan Medicine