At least 100 people have been struck down with a gastro bug after a major outbreak of norovirus on board the Indian Pacific train.

The first passenger contracted the illness almost one month ago, with the latest confirmed case just two days ago.

At least two of the 100 sick passengers were so ill they required hospitalisation, 9News has been told.

The rail service which runs between Perth, Adelaide and Sydney is still operating.

Great Southern Rail which operates the service, said it was taking every available measure to control the spread of gastro, including boosting hygiene practices, removing six affected carriages and implementing heavy-duty cleaning at all stops.

SA Health is monitoring the situation, saying it has the power to halt the train service, should it be required.