Repair to Sewer main in Court Yard – NSW

Hi, I have townhouse style apartment in NSW which has a court yard. The Sewer line for my apartment goes into the garage and joins up to 6 other units and does not run into my court yard.

Based on Sydney water maps and the fact there is a vent in my yard, there is a sewer pipe under ground which I am assuming runs under the ground within my yard. The problem is a strong smell of sewage in the yard and inside my unit. Also the ground around the Vent pipe is damp too.

Sydney water has visited but said not their problem. (Bit I am not sure)

If they are right then who need to dig up the ground and fix the problem, and then pay?  Will it be my cost, or a strata cost, or a sydney water to pay.  I have used the web site “Dial before you dig” to get a copy of the sewer plans from Sydney water. It clearly shows the my unit does not have any sewer pipe going into this main pipe across my yard.

Many thanks for ideas on who to call and speak too.

Regards Leon