First of all, when we are talking about the national government, we are talking majorly about the central government of a country. This is the highest level of a nation's governance, and it consists of people that are in highly influential, powerful, and prominent positions, such as the monarch, president, Prime Minister, vice president, head of state, and members of representatives.
The national government also includes the judiciary, the lawmakers, and probably more as the case may be in any concerned country. On the other hand, the federal government deals more with the governing system of a particular country and can determine the central government entities.
The federal government is associated more with the federation, and it includes the lower units of government like the state government. Hence, the national government happens to be the highest level government in a federal government. It all solely depends on the system adopts by a particular country.