The stench of rotten fish that Hout Bay residents have endured occasionally was caused by mechanical failure and a technical breakdown at the fishmeal factory in the harbour, according to the owners, Oceana Group Limited, and the city's health office.

While the company reported that, in one incident, it had problems with a conveyer, leading to a 13-hour delay in processing fish, a council investigation found pits had remained full of fish for about 15 hours during another breakdown days later.

According to an Oceana report on the February 29 incident, 380 tons of herring were delivered to the factory that day.

"At 7.53pm the plant experienced a technical breakdown on the off-loading conveyer. This resulted in a delay in processing until 9am on March 1," the company reported.

"The delay resulted in water storage tanks overflowing. Corrective action is that this water is pumped to the effluent system for disposal.

"The effluent pumps were not operational at the time, which resulted in water flowing back into the stormwater drains and then into the harbour. The plant recommenced production at 9am."

Oceana said it regretted the incident and said it was isolated, but on March 12, Hout Bay residents had to endure a putrid smell again.

Company spokesperson Mbuyi Mtsheketshe confirmed the second breakdown and said authorities had been informed at the time.

Lauren Runnalls, mother of a small baby, said the smell was so overwhelming she had to close her windows, despite searing heat.

"It was worse than I have ever smelled before - it burned my nose. The smell prevailed all night long and was so unbearable we could not sleep."

The city's acting director of health, Ivan Bromfield, said only two of 11 complaints received were valid.

A council report on the second incident said the plant's pre-heater motor failed about 11pm when there were pits full of fish. Processing resumed at noon the next day.

According to the council's information, Oceana had not been penalised as environmental regulations had not been broken, Bromfield said.