The 16 patients - all Victorian high school students and their teachers - spent the night in hospital but were well enough to eat a "bit of toast and Vegemite" on Friday morning after a disastrous end to their pilgrimage to South America for World Youth Day.

They were hoping to fly back to Melbourne on Friday afternoon, if the health of one of the group improved, a spokesman for Catholic Education Melbourne said.

The students and their teachers had boarded QF28 in Santiago after a two-week trip to South America for World Youth Day, which drew an estimated three million young people to Rio de Janeiro in Brazil to see Pope Francis.

But shortly into their flight, some in the group started falling ill and rushing to use the facilities. There are thought to be only eight toilets on the Boeing 747-400, which can carry up to 394 passengers in some Qantas configurations.

The symptoms gradually spread throughout others in the group. In total, 26 passengers were struck down in the mass vomiting and diarrhoea episode.

The ill passengers were all travelling as part of the same group and are believed to have been infected before the flight.

NSW Health said their symptoms were consistent with norovirus, a highly contagious infection and the leading cause of gastroenteritis in Australia and worldwide.

The students are from a number of schools in Melbourne and the regional Victorian city of Sale.

A team of paramedics met flight QF28 when it landed at Sydney Airport about 5.30pm on Thursday and treated the ill passengers. Three of them were so ill they had to be stretchered off the plane.

The plane was quarantined and moved to an area at the airport where medical staff could triage and assess the patients. The plane was also disinfected.

Sixteen passengers were then taken to Prince of Wales Hospital and St George Hospital for treatment, but a Catholic Education Melbourne spokesman confirmed all been released on Friday.

"They're all out of hospital eating a bit of toast and Vegemite and swapping war stories," the spokesman said.

In a case of very unfortunate timing, travellers signed up to a Qantas mailing list reported receiving an email on Friday morning advertising discounted flights to Santiago.

Dying with laughter that I just got an email from Qantas saying "Grab your bags ✈ USA and Santiago are on sale" HA ILL BET ITS ON SALE — Matt Gilbertson (@matt_gilbertson) August 1, 2013

Bad coincidence. After Qantas pax gastro story (no fault of QF), Qantas email special offers to that destination today. LOL — waggacrow (@waggacrow) August 1, 2013

According to NSW Health, norovirus infections are highly contagious and are a leading cause of gastroenteritis in Australia and worldwide.

Outbreaks most commonly occur in residential care facilities, hospitals, schools and child care centres, and can occur at any time of the year, but more commonly in winter.

Those who contract the infection, which is spread by direct contact, can suddenly develop vomiting and diarrhoea, and may also develop a fever, stomach pains, headaches and muscle aches. Infected people, particularly young children and the elderly, can become dehydrated.

Symptoms usually begin between 24 and 48 hours after exposure to the virus and can last for one or two days.