NSW Valuer-General David Parker has warned that falling market prices will leave home owners with overstated land values when their next assessment notices come out early next year, advising people to start collecting data of local sale prices now to challenge valuations within the allowed window.

Notices mailed out between January and April next year for some 2.1-million residential properties will form the basis for land tax payments next year as well as local rates for the next three years and owners will only have a 60-day window to challenge their valuation.