As many as 10 million people around the world may be infected with the new strain of coronavirus, a health chief says.
More than one million people have tested positive for the virus, but the true figure is much higher than that due to a lack of testing, under-reporting by governments, and some patients not showing any symptoms at all.
Australia's chief medical officer, Brendan Murphy, said the actual total could be ten times higher than the reported number of confirmed cases.
Without criticising any countries, Mr Murphy said he had doubts about some of the data produced by governments, adding: "The only numbers I have total faith in are the Australian numbers, frankly."
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He added: "Worldwide we have passed one million infections. But we believe the true number is five or 10 times as much."
The world surpassed the one million mark on Thursday evening as infections soared in many countries, including Britain, the US, Spain and Italy.
More than 50,000 people have died.
The UK's death toll stood at almost 3,000 on Thursday with just under 34,000 confirmed infections.
The US has the most confirmed cases (245,373), followed by Italy (115,242), Spain (112,065), Germany (84,794) and China (81,620).
Britain has vowed to increase its tests to 100,000 a day by the end of April.
This week it carried out 10,000 tests in a single day for the first time during the pandemic.
However, there are doubts over whether the goal of testing 100,000 people a day by the month's end is achievable.
Asked if he can guarantee 100,000 tests a day will happen by the end of April, Health Secretary Matt Hancock told BBC Breakfast: "Yes, it's got to happen."
He added: "I've got a plan to get us there. I've set it as a goal. It absolutely is what the nation needs."
He said there is still a "huge amount of work to do" to get to that point.
He said: "One of the things I tried to do yesterday was set out just some of the real-world challenges that there are.
"There are barriers, there will be bumps in the road and some of this is going to be very difficult."
Speaking to reporters in Australia, Mr Murphy said the mortality rates vary so much around the world that he believes many infections are going undetected.
The World Health Organization has called on countries to significantly increase testing for coronavirus, while some critics have also highlighted differences in how some countries count coronavirus cases.
China - where the outbreak began late last year - has only recently started counting asymptomatic cases of coronavirus, prompting criticism from some international experts.
Australia has reported almost 5,300 cases and 28 deaths so far.
