Nearly 2000 landowners should receive a letter on Tuesday from Hawke's Bay Regional Council advising their properties are on a list of potentially hazardous sites.
The sites range from former timber treatment plants and petrol stations to orchards and private sections.
The council is warning landowners before it publicly issues the list in mid-October at the direction of the Ombudsman.
Council chair Alan Dick says it has been difficult agreeing to comply with the instruction.
On one hand the Ombudsman says it is in the public interest to issue the information, but on the other the Privacy Commisioner has concerns about individual's rights being breached.
Mr Dick says the council is confident the majority of the sites are low risk.