West Africa's Ebola virus outbreak was "preventable" and is on the hands of "complacent" institutions that failed to heed stark early warnings, says an Australian aid worker who has returned from Monrovia.

Medecins Sans Frontieres project co-ordinator Brett Adamson said that like Sierra Leone and Guinea, Liberia is in the midst of an unprecedented epidemic that demands international emergency aid on a "truly, truly massive scale". But while the MSF has called for support since the start of the outbreak of the deadly virus in Guinea in June, international action has been devastatingly slow.

Catastrophe: MSF's Brett Adamson says the international response is too slow. Credit:Brendan Esposito

"We led the response and we were very clear in calling for support. We knew that it was a humanitarian emergency, that it was going to be a catastrophe," Mr Adamson said.

"Now, we're in this position of saying, 'What do we do?' It's a direct result of inaction. We asked for the capabilities of the biggest institutions in the world."