The Malagasy giant hognosed snake is not poisonous, and experts say it avoids confrontation. (Calgary Zoo)

Two non-poisonous snakes that escaped into an open drain at the Calgary Zoo have been located.

The Malagasy giant hognosed snakes were found on Tuesday afternoon just below the drain in their enclosure. A zookeeper had inadvertently left it open the previous afternoon, allowing the reptiles to escape.

"A normal procedure that was put in place seven years ago when we opened the building wasn't followed," said Cathy Gaviller, director of conservation, education and research. "We recognize that's unacceptable. It was clear the snakes had made their way into the drain of the pool within their habitat after a routine cleaning."

The female snake, Maddie, and her brother, Oscar, are being assessed by veterinary staff.

Officials had reassured the public that the snakes could not have entered the city's sewer system, and were likely hiding within the dark warmth of the zoo building's plumbing system.

Six to eight people searched through 274 metres of plumbing for the two-metre long reptiles, said zoo officials.

Parents were turned away from the African Savannah exhibit, which was closed as the zoo searched for the missing snakes. (Zulekha Nathoo/CBC)

A python, which shared the same exhibit space as the escaped snakes, was too big to fit through the drain, they added.

The Malagasy giant hognosed snake is native to Madagascar and is commonly exported as a pet because of its gentle manner. According to experts, the Malagasy giant hognosed snake rarely bites, and is likely to crawl away to avoid confrontation.

Snakes feel more secure in tight, dark spots and will likely seek out places they haven't explored yet, said Jim Dyke of J & J Reptiles in southeast Calgary, which has occasionally brought in the Malagasy giant hognosed on special order.

He said snakes are naturally good at hiding and are usually found only by luck.

Last week, experts from two zoo agencies called in by the Calgary Zoo began investigating animal care practices at the facility.

Animal rights groups have criticized the zoo after the deaths of several animals in recent years, including a baby elephant, a hippo, a wild goat, four gorillas and more than 40 stingrays.

The death of a capybara crushed by a hydraulic gate in December prompted the zoo's president to enlist the help of the Canadian Association of Zoos and Aquariums and the Washington, D.C.-based Association of Zoos and Aquariums (AZA).

With files from Zulekha Nathoo and Diane Yanko