Hamilton's landmark Fairfield bridge will be undermined if hydro-generator Mighty River Power keeps changing the Waikato River's water level, says Hamilton City Council.

The safety of the bridge - built in 1937 and used by 20,000 vehicles a day - and the erosion of riverfront sections were among concerns raised at a hearing that started yesterday.

Mighty River Power has applied for resource consent giving it even wider powers to use the river for 35 years.

Of the 184 submissions, 152 oppose the plan. The hearing is expected to last about four weeks.