One of the most important surviving Roman documents is the lex de imperio vespasiani which was used to grant emperor's powers to Vespasian. It carries the words lex which suggests it was made statutory by an assembly, yet it is suspected that it is a Senatorial decree - consultum - that has been made into law.

The English translation of the lex de imperio vespasiani:

. . . or it shall be lawful for him to make a treaty with whom he wishes, just as it was lawful for the deified Augustus, for Tiberius Julius Caesar Augustus, and for Tiberius Claudius Caesar Augustus Germanicus;

And that it shall be lawful for him to hold a session of the Senate, to make a motion in it, to refer a matter to it, to propose decrees of the Senate by a motion and by calling for a vote by division, just as it was lawful for the deified Augustus, for Tiberius Julius Caesar Augustus, for Tiberius Claudius Caesar Augustus Germanicus;

The last paragraph is consistent with Tribunacian powers. The Tribune was originally a plebian constitutional position created so the Assemblies would have a magistrate position capable of holding a session of the Senate, making plebian legislation, and being able to veto the Senate, Assemblies or Consuls. Augustus made the Tribunacian powers the center of his imperatum, making himself Tribune for life. Future emperors started dating their reign from when they received Tribune powers.

And that, when a session of the Senate is held in accordance with his pleasure or authority or order or mandate or in his presence, the authority of all proceedings therein shall be maintained and shall be observed, just as if that session of the Senate had been announced and was held in accordance with a statute;

And that whatsoever persons seeking a magistracy, power, imperium, or charge of anything he commends to the Roman Senate and people and to whomsoever he gives or promises his electoral support special consideration of them shall be taken in every election;

And that it shall be lawful for him to advance and to extend the boundaries of the pomerium whenever he considers it to be in accordance with the public interest, just as it was lawful for Tiberius Claudius Caesar Augustus Germanicus;

And that whatever he considers to be in accordance with the public advantage and the dignity of divine and human and public and private interests he shall have the right and the power to do and to execute, just as had the deified Augustus and Tiberius Julius Caesar Augustus and Tiberius Claudius Caesar Augustus Germanicus;

Much of that is the Senate abrogating its responsibilities, but you can also see the emperor's powers cutting into the aedileships and crossing both human and divine law. Which had been separated out to a fair extent in the Roman Republic. Though a Tribune's sacriato meant that violence could not be done to a Tribune without defying divine law. They were essentially sacred.

And that by whatever laws or plebiscites it has been recorded that the deified Augustus or Tiberius Julius Caesar Augustus and Tiberius Claudius Caesar Augustus Germanicus were not bound, from these laws and plebiscites Emperor Caesar Vespasian shall be exempt ; and whatsoever things it was proper for the deified Augustus or Tiberius Julius Caesar Augustus or Tiberius Claudius Caesar Augustus Germanicus to do in accordance with any law or proposed law, it shall be lawful for Emperor Caesar Vespasian Augustus to do all these things ;

And that whatever before the passage of this law has been done, executed, decreed, ordered by Emperor Caesar Vespasian Augustus or by anyone at his order or mandate, these things shall be legal and valid, just as if they had been done by the order of the people or of the plebs.

Sanction

If anyone has done or does anything in consequence of this law contrary to statutes or bills or plebiscites or decrees of the Senate, or if he does not do in consequence of this law what it is proper for him to do in accordance with statute or bill or plebiscite or decree of the Senate, this shall not be to his prejudice nor shall he be liable to pay the people anything on this account, nor shall anyone have the right to an action or a judgment concerning this matter, nor shall anyone allow an action concerning this matter to be pleaded before him.

When Caesar made himself dictator for life, and it must be remembered a dictator was a valid Roman constitutional position, he offended the ruling oligarchs of the Senate. Who killed him - in the Senate.

Augustus was smarter and did not offend the oligarchic families so easily. He stopped taking successive consulships, as this was a means for aspiring young public figures to establish their political and public bona-fides through martial success. The Tribune for life gave Augustus the permanent magistracy he needed while still have the constitutional powers to call upon and veto the Senate and Assemblies.

By the time of Vespasian, the constitutional mechanisms for imperial power were well established, and decrees from the Senate, such as this, were sufficient to create the next emperor. By Vespasian's time, policy came from the emperor and the other constitutional bodies followed that policy, unlike during the republic, when the Senate, Assemblies, Consuls and other magistracies were far more independent policy wise.