We granted certiorari to the Court of Appeals’ opinion in Star Laundry v. City of Warner Robins, 189 Ga. App. 839 (377 SE2d 709) (1989) in order to determine whether the defendants were entitled to assert the defense of estoppel in opposition to the city’s suit for amounts claimed for water and sewer services.
As the result of a new rate ordinance, the defendants’ monthly water and sewer bills increased dramatically. The defendants sought help from the mayor who agreed to a rate structure for each of the defendants much lower than that required by the ordinance. The defendants were billed at this lower rate until the city council learned of the agreement approximately a year later and put an end to it. Id. at 840. The trial court awarded summary judgment to the city in its suit against the defendants for the difference between the amount charged for water and sewer services under the agreement with the mayor, and that required by the ordinance. The Court of Appeals reversed, holding the city was estopped from recovering the amounts claimed.
The city contends the defendants are barred from asserting estoppel by OCGA § 45-6-5 which provides:
Powers of all public officers are defined by law and all persons must take notice thereof. The public may not be es-topped by the acts of any officer done in the exercise of an unconferred power.
It is undisputed the mayor had no authority to lower the defendants’ water and sewer rates. Star Laundry v. City of Warner Robins, supra at 841 (2). A governing authority may not be estopped regarding an ultra vires act. OCGA § 45-6-5; Corey Outdoor Advertising v. Bd. of Zoning, 254 Ga. 221, 224 (327 SE2d 178) (1985).
The defendants’ argument that OCGA § 45-6-5 and Corey Outdoor Advertising v. Bd. of Zoning, supra, should not apply here because the defendants relied to their detriment on the agreement established by the mayor and, had they not done so, they might have taken steps to reduce their obligation under the rates established by the ordinance (such as charging their customers more or reducing the number of customers) misses the mark. Detrimental reliance, which is *349a necessary element of any claim of equitable estoppel, see generally Bell v. Studdard, 220 Ga. 756, 760 (4a) (141 SE2d 536) (1965), is not a factor where estoppel cannot be applied against the city as a matter of law.
Unlike City of East Point v. Upchurch Packing Co., 58 Ga. App. 829 (200 SE 210) (1938), relied on by the Court of Appeals, and Quillian v. Employees’ Retirement System of Ga., 259 Ga. 253 (379 SE2d 515) (1989) where the governing entities were estopped by the action of a governmental official, this case does not involve an irregular or unauthorized method of exercising power. In City of East Point, supra, the city official who misread the meter and caused the underbilling was acting within the scope of his authority as superintendent of the city’s electric department in reading the meter. Similarly, in Quillian, supra, the state official, whose calculation of Judge Quillian’s pension the judge relied on in submitting his resignation, was acting within the scope of his authority as Director of the Employees’ Retirement System in calculating that pension. Here, however, as in Corey Outdoor Advertising, supra, the government official had no authority to take the action in question, in this case, setting water and sewer rates. Further, contrary to the defendants’ arguments, and the position of the dissent, there is no evidence the city was aware of the agreement by the mayor for a lower rate structure for the defendants, and no evidence the council members ratified the agreement.1 We do not reach the defendants’ argument, raised for the first time on appeal, that the city was negligent in failing to discover that agreement earlier.2
Accordingly, the defendants may not avail themselves of the doctrine of estoppel, and the trial court properly granted summary judgment in favor of the city.
Judgment reversed.
All the Justices concur, except Smith, J., who dissents.Several of the council members suggested the defendants contact the mayor so that he would be aware of the defendants’ problems. However, the record is clear these council members did not direct the defendants to try to work out any agreement with the mayor, did not expect the mayor to take any specific action, and certainly did not expect he would enter into an unauthorized agreement with the defendants. We are unable to conclude, as does the dissent, that the council members’ testimony is expressly or impliedly contradicted so as to create a fact issue as to ratification.
We note the establishment of water and sewer rates is a governmental function, Glendale Estates v. Mayor &c. of Americus, 222 Ga. 610, 612 (1b) (151 SE2d 142) (1966); Horkan v. Moultrie, 136 Ga. 561, 563 (1) (71 SE 785) (1911) for which the city is not liable in negligence. OCGA § 36-33-1 (b); Corey Outdoor Advertising v. Bd. of Zoning, 254 Ga., supra at 224 (3).