I agree with Diego. At the ...

I agree with Diego. At the very first stage you will need to a have a more precise definition of the scope of audit. Once you have a clear understanding of the scope of audit you can send a request for interest to 5-10 international companies. In each proposal you will receive a nice list of what will be done and what has been done already. You will just need to summarize the information provided by each company.

Theoretically you always start with the hydrolic balance (water balance) in you city. You will need to find all parts of the equitation where water supplied to the system = water used in the system + leakage + water rejected to the ground.

At this stage you will need to order a water metering system audit. At the end of this audit you will understand the status of the water / waste water metering systems. Which types of water meters do you have at present (technical, industrial, commercial, domestic (residential).  Does your current water metering system allow you to find all elements of the hydrolic balance equitation ?  If yes you will find that 100 m3 of water supplied to the system = 70 m3 ejected to the ground + 10 m3 used in the system + 20 m2 leakages. Than you can order the audit of leakages to understand the elements of the equitation total leakages = identified leakages  + non-identified leakages. Identified leakages will be results of water network accidents/ Km/month and duration of each accident. 

You can order an audit of the water /        waste water networks to understand the length of the network, characteristics of pipes (age, materials, size). You can order a financial audit to understand the pricing policy and effectiveness of the financial policy.    You can order an audit of pumping stations to understand the efficiency of your pumping equipment. You can order an audit of your water treatment facilities.