It's a fluid situation. The data wasn't complete, and since then Sierra Leone has become the "growth country" for infection, appearing to outstrip Liberia, where infection in rural areas may have dropped off.

Within the next two weeks, the CDC will announce if the half-million prediction still holds, or if the situation is better or worse.

At the centre of these predictions is Dr Manoj Gambhir, a Monash University epidemiologist, who has been seconded to the CDC's Ebola emergency response, as part of the modelling taskforce.

He arrived in Atlanta six weeks ago, just as the "half-a-million" announcement was made.

"There have been changes in the trajectory since then," said Gambhir, speaking from Atlanta. "It could be that in some countries, and particularly in Liberia, it may have slowed, at least in the rural areas.

"In the urban areas, the capital, it's still explosive. But the data coming out is not very reliable. We're relying on new modelling predictions which are about to be written up."

Gambhir, who spends 80 per cent of his time running numbers on a laptop, and the rest of the time in meetings, said that computer modelling was playing a crucial role in this outbreak because of the difficulty in collecting reliable data from the ground - because people working in the field are overwhelmed.

Gambhir is one of 10 members of the modelling taskforce. The magic number they work with is "Re" - which stands for Ebola's reproductive rate or, the average number of people that an infected person goes on to infect.

Based on their predictions of how many infections will occur, and where they will spread, the ground troops - doctors deployed by the World Health Organisation and Medecins Sans Frontieres - can estimate the medical supplies and personnel they will need, although the latter has proved to be a bitter wish list.

Gambhir said the CDC was running at a pace that he hadn't seen before.

"So much is happening. People are stretched thin and having to do a lot of work. President Obama came to visit a month ago and then called a week ago to raise the spirits. There is a sense of determination here that we will succeed."

Australia doesn't have a CDC organisation, and Gambhir said there was pressure for one to be established.

"We could be looking at emerging infectious disease outbreaks with some frequency over the next decade or so, and the Asia Pacific region should be prepared. We are not ready for it."

Australia has a "diffuse network of experts at universities and working on government, but no readily available mechanism to respond to an outbreak in a coordinated way. My colleagues will be calling for a small version of the CDC to be established".

NSW Health upgrades response to all haemorrhagic fevers

Doctors who suspect a patient is suffering any viral haemorrhagic fever, including Ebola, must phone the NSW Health Department, after changes to the Public Health Act.

The listing of all viral haemorraghic fevers, Ebola, Lassa, Marburg and Crimean-Congo, as diseases notifiable by doctors if they suspect a case escalates the NSW health system's response to Ebola.

Previously the diseases were only notifiable after they had been confirmed by a laboratory or hospital.

Health Minister Jillian Skinner amended the Public Health Act last week to allow the health department secretary to direct a person suspected of being infected to undergo medical examination.

A viral haemorrhagic contingency plan released to local health districts on Wednesday requires suspected cases to be treated as a public health emergency.

Any person with a fever who has returned in the past 21 days from known infection areas must be assessed and isolated, with medical staff to wear protective clothing, described as a fluid-repellent long-sleeved gown, face shield, mask and hood, below-knee boot covers, double gloves and waterproof apron.

NSW director of communicable diseases Dr Vicky Shepherd said Ebola is the only active outbreak, so travellers from Guinea, Liberia and Sierra Leone are subject to the new procedure.

Kirsty Needham