A female gorilla has died at Melbourne Zoo after a young male gorilla attacked her in an "unprecedented" show of aggression.

The 33-year-old female, named Julia, was the victim of the assault from 13-year-old Silverback male, Otana, on Friday. Staff monitoring the male noticed he had been showing an unexpected level of aggression towards Julia, before she retreated to a cave for the evening.

On Saturday, Julia was transferred to the vet for an examination, but her condition could not be stabilised. Head vet Michael Lynch told Seven News Julia had significant muscle damage caused by biting and hitting.

"The vet team examining Julia found that she had experienced extensive trauma and significant complications resulting from the aggression on Friday," Zoos Victoria said in a statement. "The team worked to stabilise her condition into the early hours of Sunday, but unfortunately she passed away at 3am today [Sunday]."

A group of school children witnessed part of the violence on Friday morning. Belinda Carroll told the Herald Sun her daughter, five-year-old Bella, and 17 of her classmates from Altona Green Primary School saw Julia being attacked at 9:20 a.m. when they visited the enclosure.

“Her lower fur was all bloody and red and you could see a gash," Caroll told the publication. "Julia would run away and then the male would come back and pounce on her, slapping her again. It was really frightening."

The zoo wrote in the statement Otana appeared to have settled well into the zoo with the five female gorillas after arriving in 2013 from the UK, and up until this point, the female gorillas had responded in the submissive way male gorillas expect. Yet, despite being 13-years-old, Otana has still not been acting like a male leader and has previously displayed intermittent signs of aggression, although not this severe.

On Sunday, the zoo announced Julia's death, stating staff were devastated by the death of their beloved gorilla, who came to Melbourne Zoo in 1997. Those who worked with Julia have been offered counselling.

Otana has been removed from the group and the zookeepers are assisting the other gorillas through the tragedy, due to the impact such a death will have on the social makeup of the group.