"Though the law is the same for all, all are not equal in front of the law," said Saeverud, justifying the fine.
"It is therefore correct to issue a fine in order to uphold the general public's trust in the rules on social restrictions," he said.
Police said Solberg and her husband, Sindre Finnes, made the decision together to hold a celebration and picked the restaurant, with Finnes taking care of the practical arrangements.
Though police said he had broken the law as well, he was not fined. The restaurant where the celebration took place was also found to have violated the law but it was not fined.
"Solberg is the country's leader and she has been at the forefront of the restrictions imposed to limit the spread of the virus," said Saeverud.
The prime minister's office was not immediately available for comment.
Solberg, who faces elections for parliament in September, has championed strict rules to curb the spread of the coronavirus, resulting in some of Europe's lowest rates of infection and deaths.
But the country saw a rapid rise in infections in the first quarter of 2021, led by more contagious variants of the virus, forcing the government to tighten restrictions in late March.