One of Adelaide's main beaches has reopened after a pollution scare shut down a nearly half-kilometre section of it last week.

On Friday, the 400-metre section - located just north of Y-Steps - was closed and users were warned to avoid it when unchlorinated water from a wastewater treatment plant was released into the ocean.

Up to 6 megalitres was released because of a fault with the plant's automated chlorination system and monitoring alarm.

"A fault in the automated chlorination system was discovered by staff completing routine checks," SA Water's senior manager of production and treatment Lisa Hannant said.

"They immediately repaired it and returned the plant to normal operation."

The affected section was still closed Monday morning, but now has the all-clear (Photo ABC News: Spence Denny)

SA water said the plant resumed normal operations on Saturday morning but the temporary closure remained as a precaution.

While the water which caused the scare had been cleaned and treated, the lack of chlorine meant it was not disinfected.

SA Water explained that chlorine disinfection is the final stage of treatment, killing any micro-organisms that remain after processes that cleaned the water and removed solid or organic matter.

Without it, people who came into contact with the water could have experienced diarrhoea and vomiting.

© Australian Broadcasting Corporation. All rights reserved.