Refusal to fix defects

I bought a property off plan from TYRIS/Ruby Mountain at St Aidans in Barbeque Downs. This is the same developer for Morganwood in the same area. Once the property was completed the developer trenched through the garden to create a drainage system. When I requested that they run the drainage underground they out right refused besides the fact that it made the garden unsightly and under heavey rain my garden becomes a drain. There in similar issues in a few other units with this developer. Be careful when you buy anything from them.