Auckland's busy Victoria Park flyover will be closed for maintenance from 10pm, starting tonight, to 5am on weekdays until 18 March.
The Transport Agency said the bridge's barriers would be replaced and energy-efficient lights installed.
The bridge, part of State Highway 1, carries traffic heading south.
The agency's highway manager for Northland and Auckland, Brett Gliddon, said while the flyover was closed, signs would direct drivers through the city and back onto either State Highway 1 or 16 at the Hobson Street on-ramp.
"We'll have pretty well signed detour routes, they'll just come off at Fanshawe Street and they'll just roll through town and back on at Nelson Street, so there's a couple of streets they'll have to run through the city just to get up to there."
Mr Gliddon said the detour should only add a couple of minutes to journeys.