WARNING, dog lovers: the plain-clothes poo police are watching.

After an increase in complaints, undercover City of Sydney rangers will be pinging careless dog owners a $275 on-the-spot fine for failing to bin their pup's poo.

"Stepping in dog waste can ruin a family picnic or a morning jog and many parents have expressed concern about hygiene issues for young children," Garry Harding, the City of Sydney's director of services, said yesterday.

"Uniformed rangers have found dog owners are generally responsible during patrols, but some irresponsible owners … change their behaviour when rangers leave. Those owners are now on notice to be responsible or face a fine."

The patrols will start in mid-July in all areas - including the city's 33 off-leash parks.