"Everyone is doubling, tripling, quadrupling whatever they were planning," said Caroline Haga of the Red Cross in Beira, Mozambique, referring to supplies and aid workers.
"It's much larger than anyone could ever anticipate."
The European Union released 3.5 million euros (£3 million) in emergency aid, and the UK pledged up to £6 million. Neighbouring Tanzania's military airlifted 238 tons of food and medicine.
Matthew Pickard of the humanitarian organisation Care said the response to Idai has been similar to previous natural disasters.
Local authorities and international non-governmental organisations worked their way to the area in the first days, with additional aid destined to arrive soon after.
The slow-moving catastrophe of the flooding and the inability to access some of the hardest-hit areas has limited the ability of some to see the scale of the cyclone, but aid will spike as the details become clearer, Mr Pickard said.
"Over the next few days we'll learn just how big it is," he said from Malawi.
"These are countries that are not usually making headlines and they're making headlines.
"With the story comes people's intent to respond empathetically."
Sacha Myers of Save the Children, speaking from Mozambique, described rising floodwaters, "rivers and dams bursting their banks" and a death toll in the hundreds and rising.
She was awaiting the arrival of a cargo plane carrying 51 tons of emergency supplies, but said getting them where they needed to go remained difficult with roads washed away or submerged and few options for storage in dry areas.
"We're having an unfolding crisis that's getting worse and worse," she said.
The United Nations is deploying resources too, deputy spokesman Farhan Haq said, but logistics remained challenging and the hardest hit areas remained inaccessible.
As better data emerges from the disaster zone, donors will be standing by to make money and other resources such as medicine available, said E Anne Peterson of the non-profit health organisation Americares.
"It's early and a really big disaster gets attention fast, and the more media covers it, the more people realise there is a need and the more likely we are to see them getting engaged," she said.
On Tuesday, International Development Secretary Penny Mordaunt said a team of experts was on the ground in Mozambique helping to coordinate the UK's response.
Ms Mordaunt said: "I have made £6 million of UK aid available to help meet the immediate needs of people who have lost everything.
"We have deployed a UK team of DFID experts who are now on the ground in Mozambique helping to co-ordinate the UK's response to this disaster, and we hope to have vital UK aid supplies in the region shortly.
"We stand ready to scale up our support if needed.
"The images of loss and devastation following this deadly cyclone and extreme weather are shocking.
"The people of Mozambique, Malawi and Zimbabwe should know that they are firmly in our thoughts at this difficult time, and that the UK stands by their side."
Filipe Nyusi, the president of Mozambique, said more than 1,000 people are feared dead in the country four days after the cyclone struck.
Entire villages were submerged and bodies left floating in the floodwaters in the wake of the natural disaster.
In September 2017, British residents were killed, homes were destroyed and infrastructure was damaged when Irma and Maria, two of the most powerful hurricanes for decades, struck Barbados, Antigua and Montserrat.
The Ministry of Defence has said it is supporting other Government departments to ensure that similar devastation is not repeated.
RFA Mounts Bay has been in the Caribbean since Irma struck and will remain there until 2020 to help restore the islands to normal.