I was searching on google and I found this at : http://shrestha-history.blogspot.com/

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" Shrestha or Shresta (Nepali: श्रेष्ठ), is a surname belonging to the community of people in Nepal and India and trace their origin to Northern-Indian roots. They originate from the Indian state of Uttar Pradesh in India. During the reign of the Shah kings in Nepal, Shresthas used to work mostly as laborer in India. Even amongst Shresthas, there is a differentiation of caste, called 'thari', the difference being mostly between "panch thari" (5th level) and "ccha thari" (6th level). It is speculated that there is an additional "das thari" (10th level).

Often people from various parts of Nepal who are considered of lower caste (Sarki, Damai, Chyame etc.) have changed their last name to 'Shrestha' and that is the main reason why the number of people bearing 'Shrestha' as their last name has grown exponentially in the last couple of decades. That is also the reason for above 80 percent of people who have 'Shrestha' as their last name can't speak the Newari language.

It is also believed that another caste "Pradhan", originally pronounced as "Pa-Ra-Dhaan". Which means, rice carrier, also came from central parts of India. During Rana rule, Pradhans from different parts of Nepal started to pour in to Kathmandu and mixed themselves with Newars living in the valley. They were called "Pa-Ra-Dhaan" because their job was to carry huge sacks of rice on their backs, to transport from local Newars in Kathmandu to outside of the valley. "