Austin, Tex., arguably has the prettiest sewage-treatment plant in the nation, but the city may have to pony up more of the construction costs for the striking Aztec-style building.

Auditors for the Environmental Protection Agency are contesting more than $500,000 in expenditures for such "esthetics" as native-stone walls, a 1,200-square-foot visitors' lobby with a sunken lounge and a $156,000 municipal meeting room deemed to have nothing to do with treating sewage.

According to EPA regional officials in Dallas, the agency's auditors concluded after a routine run through the books last year that the federal government shouldn't pay its 55 percent share of $850,000 in questionable expenses for the $31 million Walnut Creek plant.

The findings put EPA officials in an awkward position. They had approved the elaborate design as a way to help counter neighborhood opposition to expansion of the 15-year-old treatment plant, and figured they'd saved the government money in the process. Moving the plant would have cost an additional $8 million, according to Karen Brown of the EPA. INSIGHT, IN HINDSIGHT

Former EPA administrator Anne M. Burford, who left under fire more than two years ago, says she now doubts the Reagan administration's commitment to the EPA and wishes she hadn't tried so hard to get along with the Office of Management and Budget.

In a speech to a hazardous-materials conference in Philadelphia last week, Burford cited President Reagan's failure to give an address on environmental issues and his appointment of an EPA administrator who is "not only a Democrat, but a Democrat who is a bureaucrat."

"I don't see a commitment to the mission of the EPA," Burford was quoted in the Bureau of National Affairs weekly publication, Environment Reporter.

Burford, sounding a bit like the congressional critics she fought with so long, also said it was a mistake to cooperate with the regulatory overseers at OMB, who she said are subject to "back-door lobbying from industry."

"EPA flatly is in an adversarial relationship with OMB. I don't know if that is just because David Stockman runs OMB and David Stockman has always been hostile to EPA," Burford said.

For the record, current EPA Administrator Lee M. Thomas has been a state or federal employe for 18 years, including four with this administration, and is not a Democrat. "He's a registered voter affiliated with no party," an aide said.