World Vision embarked on a relief operation after torrential rains swamped western and southern districts, causing mudslides which have claimed at least 151 lives.
National Director for Sri Lanka, Dhanan Sennathirajah, told Premier: "The biggest challenge right now is access to the areas [affected] because some of them are in mountainous terrain and most of the roads have got washed away or they are under five or six feet of water."
More than 100 people are still missing, while the head of the country's search and rescue operation Major General Sudantha Ranasinghe said crews are not expecting to find more survivors.
Dhanan Sennathirajah went on to say: "The extent of this disaster to way too much for any government to handle... because 15 districts are affected out of 25 districts in the country. About five or districts have been severely affected."
With more rain expected on Monday, the government said more than 100,000 people forced from their homes have taken shelter at 339 relief centres.
The United Nations says it is assisting, in response to an appeal by the Sri Lanka government, while the US has joined India and Pakistan in offering support.
Dhanan Sennathirajah added: "It has been a bit of terrible situation over here because... we haven't had this kind of rainfall, flooding, loss of life for the last two decades."
With some forests being removed to make way for tea and rubber crops, mudslides are commons during monsoon season in Sri Lanka. More than 100 people in the centre of the country died during a landslide last May.
Click here to hear Premier's Alex Williams speaking with World Vision's Dhanan Sennathirajah: