Dominic Dwyer, who is a microbiologist at Westmead Hospital in Sydney, was part of a 10-member team of experts sent to China to explore the origin of the coronavirus.

"It was actually very complicated and very tense. There is certainly political pressure, and I think that pressure is actually more on the Chinese side than it is on us," he said.

"We go there as an international group and we're not part of the WHO, we're just independent experts. So there's clearly political pressure on Chinese authorities and WHO, I guess.

"So it was apparent and there were people from foreign affairs in China attending our meetings and so on. It was at times tense, but always very well mannered and good humoured."

Some key data may not have been handed over

Mr Dwyer said the team had requested their Chinese counterparts to do certain work before they met.

"When we got there, they'd done that, including some very big amounts of work such as reviewing 76,000 case reports from 130-odd facilities in Wuhan, trying to identify early cases," he said.

"So they did do some enormous amounts of work quickly. On the other hand, whether we got all the data, of course, is uncertain.

"And it was sometimes difficult to understand how they came to certain conclusions, and there were differences of opinion over that."

The ABC understands requests to test old waste water samples in Wuhan pre-December 2019 for early COVID-19 traces were knocked back, as they were told all samples were discarded.

It is believed the team were also advised that the 2019 blood donation samples in the city could not be tested due to laws about accessing them.

"I think some of the investigation around the very early cases was hard to get a hold of, and there was a lot of back and forth about that," Mr Dwyer said.

"What you really want to do is get details on each individual case and go through them rather than just being given the overall results by them.

"So there was quite some tension around doing that."

Investigation did not solve coronavirus mystery

The probe has been at the centre of a geopolitical storm, with China's government supporting efforts to suggest the virus originated abroad while the former Trump administration continually blamed China for the global pandemic.

The visit by the WHO team took months to negotiate after China only agreed to it amid massive international pressure at the World Health Assembly meeting last May, and Beijing has continued to deny calls for a strictly independent investigation.

The WHO team, along with Chinese experts, held a press conference to announce the conclusion into their fact-finding mission earlier this week, revealing that the investigation had uncovered new information but had not dramatically changed the picture of the outbreak.

China's head of the COVID-19 expert panel, Liang Wannian, said it believed the disease may have originated from a zoonotic transmission — from animals to humans — which then evolved.

But he said the reservoir host was yet to be identified.

Mr Dwyer is now in hotel quarantine in Sydney but said while preliminary findings had failed to conclude anything clear about how the pandemic started, the mission was worthwhile.

"The WHO team had never seen this data before, even though they had asked China for this data various times over the last year," he said.

"How people turn this into their own political gain or whatever, I don't know, but it'll be complicated.

"But I think the call by Australia to have this [investigation] was absolutely sensible."

Story By | Bill Birtles