The fallout from the Ruby Princess continues with NSW Health releasing a statement tonight defending its response to the coronavirus-riddled cruise ship.

In a lengthy statement, NSW Health revealed 342 of the state's positive coronavirus cases had come from the ship.

"Transmission of COVID-19 amongst these passengers could not have been prevented by NSW Health staff," a spokesperson said.

"No cases of COVID-19 were identified on board the ship before it docked.

"The vast majority of these passengers reported they did not develop symptoms until after leaving the Ruby Princess."

NSW Health said all passengers were advised to self-isolate for 14 days after leaving the ship.

Secondary transmission from the ship has been limited to 11 cases as of today.

"International experience shows COVID-19 can rapidly spread among passengers if left on board, so self-isolation at home is a much safer option than leaving passengers on board," NSW Health said.

Ruby Princess was assessed as "low risk", according to NSW Health however influenza on the ship had been found.

"Rapid influenza tests identify only a proportion of people who actually have the infection, meaning some people return a negative result even though they are infected with the flu. The illness and test results identified on board was consistent with influenza," NSW Health said.

"This is reflected in email correspondence between NSW Health and the ship’s doctor on the Ruby Princess who confirmed influenza was circulating on the cruise.

"However, in two sick patients referenced in the email, although they had tested negative to influenza, the cause of their respiratory infection was consistent with influenza for which they were receiving treatment."

The state health department said not enough people were showing symptoms for it to be classified as an outbreak.

"Under the Commonwealth Department of Health cruise protocols, an outbreak is defined when ‘more than one per cent of the ships total passengers and crew have an influenza like illness’," NSW Health said.

"The Ruby Princess had 2647 passengers and 1148 crew. The ship reported to NSW Health there were 104 acute respiratory infections of which 36 people had presented to the ship’s clinic with influenza like illness during the cruise and its numbers fell short of the definition of an ‘outbreak’."