Depends what you are wanting to do with the site.... if there is an existing house on site and you are not planning on subdividing it, change its use or undertake earthworks then it may not be an issue. However, any future development onsite would require site contamination assessment against National Environmental Standard either via a PSI or a DSI. Council's consent maybe required under NES, and disposal of contaminated soil can be very costly. There are only two landfills in Auckland can take that kind of soil.. If PSI confirms it is below the permitted standard against NES, then you don't really need to worry about it.