School cleaners have been warned against accepting a job promoted as a "great opportunity to earn some good money" to clean a quarantine centre for the crew of the Diamond Princess cruise ship in Japan.

The job offer involves spending six days of cleaning in response to one of the largest coronavirus outbreaks outside China. More than 630 people aboard the ship have been infected with coronavirus, including two who died.

Cleaners received a text message on Monday which said a "really great opportunity has arrived".

The United Workers Union said members who work as public school cleaners for the Broadspectrum cleaning company were approached on Monday with an offer to work for six days in Japan. Some workers said they were told by supervisors they would be cleaning the Diamond Princess.

One, who works in a NSW public school, asked if the supervisor was joking and what compensation they would receive if they contracted the virus.