The international development agency said urgent humanitarian aid was needed for millions who are in dire need of water, food and shelter.
"Pakistan is facing a humanitarian crisis," said Robin Greenwood, Christian Aid's head of Asia and Middle East division.
"Thousands of people are still waiting to receive assistance. It is crucial to get humanitarian aid to the people and the places that need it most."
At least 1,500 people have been killed and 3.2 million affected in the worst floods witnessed in several decades.
Christian relief organisations Tearfund, Church World Service (CWS), Norwegian Church Aid and the German agency Diakonie Katastrophenhilfe have been distributing food packets and temporary shelter kits in the affected areas.
CWS Pakistan team members report an increasing need for assistance and say blocked roadways and communications continue to hamper aid workers from reaching cut-off survivors with vital food and supplies.
Reporting from Sibbi, Balochistan, CWS Pakistan Senior Project Officer Saleem Dominic, said, “Food is the initial, immediate need, with hygiene and shelter next. Houses are completely destroyed and flood water remains in the houses, which poses health risks."
CWS is the first organisation to distribute food in Sibbi. Dominic said flood survivors were "very grateful" for the help.
Based on assessments conducted by its staff and local partners, CWS plans to provide emergency assistance to 70,000 people in Swat, DI Khan, Sibbi and Kohistan. It includes food assistance to 35,000 people, emergency shelter supplies for 17,500 people, and mobile health access for 17,500 people in Mansehra and Swat.
Meanwhile, health experts fear an outbreak of water-borne diseases such as cholera in the northwestern region of the country.
The World Health Organisation (WHO) said the health of hundreds of thousands of people is at risk, with a high threat of water-borne disease outbreaks.
WHO has sent large shipments of medicines and supplies to treat people for diarrhoea, respiratory infections, wounds, and other health conditions.
To ensure people have access to clean water, WHO has also provided 102,000 aqua tablets and 4,600 water purifying sachets to health facilities in Peshawar and Nowshera.