My Prius 3rd Gen 2011 1800cc is also giving bad mileage in city ie 15kmpl. Although 23-24kmpl on Highway.
Reason for bad mileage in city could be the traffic condition, for instance I alway stucked in traffic during rush hour and peak time where lots of idling and slow speed, that how prius battery drain quick and Petrol Engine starts to charge prius battery. That how mileage goes down.
Although I have driven my prius in Lahore and Islamabad city, I have been getting 19kmpl.
You should take ur prius on highway for long drive and make sure meter and average is reset and then check how much average you are getting, if ur getting good average on Highway, then most prolly city is culprit due to traffics..
Although you should consider Bilalalam advise too..