A child's bowel movement in a swimming pool that led to a pump failure and sent chlorine into a nearby creek has cost Lake Macquarie council $15,000.

The NSW Environment Protection Authority issued the fine after the West Wallsend Swim Centre discharged 16,000 litres of chlorinated water into the creek next door in April.

Council fined over faecal faux pas: West Wallsend Swim Centre.

A council spokeswoman said the backwash was intended to treat a faecal contamination after "a child who was not toilet-trained used the pool without a correctly fitted aqua nappy".

Swim centre staff followed a "rigorous procedure" of evacuating the pool, chemically treating the water and back-washing the filtration equipment to remove pathogens, the council said, in line with NSW Health guidelines.