Most of the city of Saint John remains under a boil water order following a water main break early Tuesday morning.

About 45,000 people are affected by the order for the area east of the Reversing Falls bridge, including a hospital that virtually shut down for the day and a school that sent students home early.

The order is expected to remain in effect until at least Friday, according to city officials.

This is the sixth boil water order in the city so far this year; the second one in the past month.

The age of the city's infrastructure is at least partly to blame, said Kendall Mason, deputy commissioner of Saint John Water.

Saint John boil order

Everyone east of the Reversing Falls bridge in Saint John is subject to a boil water order issued by the health department until further notice. (CBC)

"The vintage of the pipe is approximately mid-1800s. It appears it could be a vintage 1873 vintage cast iron pipe, so obviously the vintage is one of the considerations in the failure," he said.

A 0.6-metre wide pipe near the Lakewood Heights pump station burst at about 4:30 a.m. Due to the break, there was an increased water flow at the Latimer Lake treatment facility, resulting in insufficient chlorine being added to the water.

The boil order was issued by the Department of Health and will remain in place until two consecutive tests, taken a minimum of 24 hours apart, show that the water is safe to drink.

Mason says the affected water main is one of the ones slated for replacement under the city's massive Safe Clean Drinking Water program.

The system upgrade, which recently secured nearly $115 million from the federal and provincial governments, is expected to be completed in 2018.

Low pressure forces closures

Tuesday's break and related low water pressure forced the near closure of St. Joseph's Hospital.

The urgent care centre, operating rooms and out-patient clinics were all closed until further notice, Horizon Health Network officials said in a statement. Only the specimen lab, urology and diagnostics remained open to the public.

Patients requiring care were asked to visit a primary care clinic or the Saint John Regional Hospital's emergency department instead.

On Tuesday night, St. Joseph’s Hospital said its operations will return to normal Wednesday morning.

Hospital staff are used to boil water advisories and have procedures to follow, executive director Brenda Kinney told CBC News. But a sudden loss of pressure cut off water completely, she said.

"We had no water so we had some serious infection prevention control issues, no ability to flush the toilets, no drinking water, no drinking water at that particular time — certainly lots of impact from having no water," said Kinney.

The Saint John Regional Hospital had extra staff on hand to deal with patients being diverted from St. Joseph's, she said.

Forest Hill Schools also closed for the day due to water pressure problems. The K-8 school on the city's east side sent its approximately 675 students home at 11:15 a.m.

Officials advise that micro-organisms in the water may make people sick and are a particular concern to those with weakened immune systems.

The city and health department advise people to use bottled water, or bring water to a rolling boil and let it boil for at least one minute, then let it cool before using.

Boiled or bottled water should be used for drinking, brushing teeth, making ice, juice, coffee, tea or washing vegetables that will not be cooked.

It is safe for people to shower, bathe and wash dishes in hot, soapy water. It is also safe to use a dishwasher.