A Board of Inquiry has begun hearing the Transport Agency's resource consent application for a proposed section of highway north of Wellington, which is part of one of the Government's roads of national significance.
The McKay's to Peka Peka expressway is a 16km stretch of road estimated to cost more than $600 million. It will require the removal of about 30 houses.
A total 725 submissions have been filed regarding the application - 302 in favour and 408 opposed.
The agency began outlining its application to the board of four commissioners on Monday.
It argues the road would enhance economic productivity by improving journey times from Wellington to and through the Kapiti district.
Action to Protect and Sustain Our Communities secretary Marie O'Sullivan says the expressway will cause air and noise pollution affecting the health of thousands of people living within 200m of the road.
"We've counted 1360 dwellings within that area, and that is in contrast to the existing State Highway 1 where we counted ... approximately 104 dwellings that are within 200m of the road."
The Transport Agency says the expressway will not pose a health risk.
Resident Loretta Pomare says the road will pass within 50m of her house, and her property's value has halved.
"I've just had an appraisal done and it's worth 50 percent of its original value."
Opposition to the road from the Green Party and lobby group Save Kapiti is likely to challenge the cost-benefit ratio of the road and its environmental impact.
The hearing is expected to last several days and a decision is due by April next year.