Rav Wozner(Shevet HaLevi 2:58):Concerning someone who works for the tax department and he discovers someone cheating the government and is required by law to report it to the justice department. He wants to know whether he is considered an informer (moser) according to halacha or do we say that “the law of the land is the law” and thus he would not be an informer? Answer: Concerning the issues of taxes – there is no halachic authority who denies that this is included in the principle “the law of the land is the law.” This is true even for those who disagree with the Rema (C.M. 369:8) as to the nature of “the law of the land is the law.” See the Shach and Levush and the responsa Hashiv Moshe… Concerning the issue of reporting the tax cheat to the government see Bava Metzia (83b) concerning R’ Eliezer the son of Rav Shimon bar Yochai. The gemora reports that he reported thieves to the government. This is proof that where the government has authorized a Jew to report thieves that it is permitted. Even though he was criticized “how long are you handing the people of our G‑d to be killed” – because the punishment for thieves in those days was death. This is relevant also for a similar criticism from Eliyahu Hanavi to R’ Yishmael which is reported in that gemora. However the actual halacha seems that even when it results in the death penalty it is considered “the law of the land is the law.” See the Ritva on that gemora which is found in the Shita Mekubetzes. The Be’er HaGola (C.M. 388) writes that it has already become accepted practice that the leaders of the community supervise that there should not be any fraud or deception against the secular government. The community leaders have announced that it was permitted to publicize and reveal those men who were cheating the government. A person who wishes to escape paying taxes owed to the government and another Jew reveals this – this is not considered the crime of informing. Even though the Rema states that revealing this information is bad because it is like returning a lost object to a non‑Jew – but that is only concerning an individual non‑Jew. But that which is applicable to the government and the tax auditor was appointed to discover fraud – there is no prohibition in revealing the fraud. However it is best if a person should not work as a tax auditor which requires revealing this type of information. Even though revealing the information is permitted – it is not a pious thing to do as we see from the Yerushalmi. Also look at the Responsa of the Alshech who states that a person is not considered an informant for those things required by the law of the land….