Melbourne Zoo will review its emergency procedures after the fatal shooting of an endangered gorilla and two lions at overseas zoos.

The western lowland gorilla was shot at Cincinnati Zoo on the weekend after a four-year-old boy climbed into the moat of the enclosure and was approached by the gorilla.

Zoo staff fired a fatal bullet rather than a tranquiliser dart because of concern over the time a tranquiliser would need to take effect.

Melbourne Zoo general manager of life sciences Hans van Weerd​ said tranquilisers could take up to five minutes to take effect and while firearms were a last resort, sometimes there was no alternative.

"When it comes to the crunch, people come ahead of animals," he said. "But what we try to do is see if there is something in between so both animals and people are safe."

Zoos Victoria chief executive Jenny Gray said the zoo community felt for their Cincinnati peers in what was a devastating scenario.

"They were faced with a situation that we always prepare for but hope we never have to experience, to be forced to shoot a much-loved animal in order to secure the safety of a visitor," she said.

The Cincinnati gorilla shooting follows the deaths of two lions at a zoo in Chile on May 22. The lions were killed after a 20-year-old man entered their enclosure and stripped naked in an apparent suicide attempt.

A select group of Zoos Victoria staff have firearms training for emergency situations. They are on duty when animals such as gorilla, lion or tiger are moved for a vet check or surgery.

Following the US and Chilean incidents, Ms Gray said all zoo managers had been asked to conduct a visual check of all dangerous animal enclosures, to review emergency response times and to ensure all licensed shooters underwent a drill in addition to the scheduled quarterly drills.

"Any organisation that operates in potentially dangerous situations has to be prepared and zoo staff are well trained to respond to a variety of potential emergencies," Ms Gray said.

Melbourne Zoo, Australia's oldest, has had its share of enclosure invasions by members of the public.

There have been unscheduled visits to the lions' den and the gorilla enclosure - and even short-term escapes by enterprising primates. But to date, no trespassing incidents have resulted in the death of an animal.

In 1989 there were two incidents involving members of the public entering animal enclosures at Melbourne Zoo.

In one, a man who climbed into the lions' den at night was found mauled to death in the morning. The second incident involved a man jumping into the gorilla enclosure and attacking an adult female and taunting the young male, Mzuri.

In the second incident, zoo staff were able to separate the man from the primates by encouraging the gorillas inside and securing them in a night den. When the man followed he was secured in a separate den until police arrived.

In 2004, lions were removed from their Melbourne Zoo enclosure after a man entered the den. He was unharmed and later arrested.

In recent years, a Sumatran orang-utan and gorilla escaped their Melbourne Zoo enclosure. Both were re-captured after being tranquillised.