The UN estimates that as many as eight million people have been affected by the 7.8 magnitude earthquake.
McGoldrick said his agency's main focus now was to reach the many affected people in remote, hard-to-reach areas.
The April 25 earthquake killed more than 7,800 people and injured thousands more. The government figures show that tens of thousands of houses have been demolished and assistance is immediately needed for 400,000 families.
"And we need to do so urgently, so that people have roofs over their heads and their other urgent needs are addressed before the monsoon season starts," he said.
The monsoon rains generally start in the second week of June and often trigger landslides in the mountains and flooding in the southern plains.
But roads are already blocked the landslides triggered by last month's earthquake and there are fears that rain hitting soil loosened by the earthquake could easily trigger more landslides.