Lake Macquarie City Council has placed a "sea level rise" notation on section 149 certificates of about 10,000 properties, but most owners do not know it.

The notation warns that the Lake Macquarie Sea Level Rise Preparedness Adaption Policy applies to the land.

The notation "generally applies to land that fronts the lake or Swansea Channel" that sits up to three metres above sea level, a council statement said.

The council has not written to property owners advising of the change.

Newcastle property valuer Bob Dupont said solicitors for people buying properties would probably be the first to know of the notations.

"The solicitors might notify the purchasers and it will probably make them think twice about buying," Mr Dupont said.

"A few sales may fall over in really low-lying areas."

Mr Dupont said property values fell around the old Pasminco smelter in the 1990s, after the council issued section 149 notations warning of lead contamination.

He said the notations led banks to restrict finance on such properties.

"When the finance industry takes notice [of sea level rise notations] that's when there will be a drop in values," he said.

"In my opinion, that will eventually happen and probably for good reason."

A council statement said the policy did not prohibit development.

"It requires the issue of sea level rise to be taken into account when new development is planned on the land," the statement said.

The policy applied to land that was "low lying and may be vulnerable to the impacts of sea level rise".

"It includes all land, including reserves and vacant land."