Asbestos problems were mentioned in 2011 in the Senate:
http://safedesign.wordpress.com/2013/05/31/nbn-asbestos-rais...
Beyond that anyone who had looked at plans from the 1940s to 1970s would have known it was there:
http://safedesign.wordpress.com/2013/05/31/nbn-asbestos-know...
Either the NBN people were completely incompetent, which is highly unlikely, or the Asbestos problem was raised much earlier internally, probably as soon as people became serious about the project.
Besides, the asbestos in the pits* would have to have been cleaned up some time anyway.
* (was NOT unforseen, my mistake)