Pipes made from the deadly material were discovered in March this year, during construction of the access road to the new council car park, next to Castle Mall.

In October, a second cache was found in the same car park and a month later workers discovered old, bonded sheets of asbestos fibro that had been discarded in nearby Cutlack Walkway.

Workers have recently uncovered a fourth cache while building a retaining wall. The deadly material was lagging around high-voltage electricity conduit cables which run throughout the council car park.

Baulkham Hills Shire Council's group services manager Mike Lathlean said an occupational hygienist had conducted a risk assessment of the site and an expert had been employed to dispose of the dangerous product  responsible for more than 40,000 deaths since the 1970s.

``It is not uncommon in any major project to come across it, given the widespread use of the product,'' Mr Lathlean said.

``We have standard operating procedures in place to deal with it. The public would certainly be advised if there was any risk. It only becomes an issue if the fibres become free. That hasn't happened, it has all been dealt with.''

Greg Hinze, principal of Castle Hill Adventist Primary School, which neighbours the work site, said: ``I am confident in council's ability to handle it. I haven't been advised about it, but the classrooms are not close.''