Building over an easement / council giving wrong info.

I purchased a property in NSW around 18 months ago that included a house that needed renovating and a 10m wide block of land ona seperate title .
Before purchasing the property i visited the local council on three occasions and spoke to varoius member in the devolpment sector  to enquire about any building restrictions,the location of the sewer , stormwater etc . I was told that there was no stormwater easement present and that i would have to drain back to the street . I then accounted for the extra costs associated with building on bearers and joists  to allow me to drain back to the street , as the block slopes towards the rear.
There is no easement registered under the title of the land
After purchasing the property a friend pointed out that there was infact a stormwater drain that appeared to run through the block . At this point i went back to council insisting that there was a drain exsistant .After initially dismissing the possibility   he was able to do further research and locate a map of the stormwater easment which cut of the top 2/3 of the block .
In conjunction with the building company  we decided i could build a double story home at the rear of the home that would completely miss the drain .
At the stage where my plans are ready to be lodged into council the building company decided they needed to locate the exact poistion of the drain in case  it would inhibit building process . The plumber located the drain and found that it infact covers the entire block from corner to corner . 
The building company then got in touch with the devolpment engineer ( who tried to argue the existance of the drain )at the local council who said it could not be built  over and i would have to relocate the drain at my expense , which would only leave me essentially a  6m wide area of which i could build and a hefty bill.
what are my options ? council denies any responsibiltiy for thier mistakes 
Research tells me that i wont be able to build over an easement in most cases  ,are the any exceptions ?
Should council take responsibility ? or is the block worthless now ?
HELP PLEASE !
Thanks in Advance