One of Auckland's fastest and busiest roads has had its speed limit lowered by 10km/h.

Auckland Transport officially introduced a 60km/h limit to Ian McKinnon Drive through Newton on Sunday although signs have been up for several days urging drivers to slow down.

Traffic was previously allowed to clock 70km/h on the sweeping 1.3km link between Dominion Rd and the city centre via Upper Queen St.

Although the road was built almost to motorway standard for the 30,000 vehicles that use it daily, and includes long downhill sections in both directions, it lacks a median barrier and has become notorious for crashes on its main bend.

The reduced speed limit follows other safety measures, including cycle lanes and traffic lights at the Piwakawaka St intersection.

But Auckland Transport pulled back from an earlier proposal supported by the previous Eden-Albert Community Board to lower the limit to 50km/h.

The council transport organisation was unable on Friday to explain why it had not following the original proposal, by the former Auckland City Council.

Waitemata Local Board spokesman Christopher Dempsey, who chaired the previous board, said he was disappointed the limit was not made the same as the 50km/h in force in Dominion Rd but accepts it would have been a challenge to get it down that far. That was because Ian McKinnon Drive had physical characteristics of a motorway, and any greater reduction would require engineering changes.

Auckland Transport communications manager Sharon Hunter said a safety review found the number of crashes along the road increased significantly after the limit was raised from 50km/h to 70km/h in 1999.

There were 58 crashes in the five years before then, rising to 90 in the five years afterwards, including one fatal smash. They included more than treble the number of loss of control crashes occurring on bends.

"The types of crashes along Ian McKinnon Drive are consistent with the speed limit being too high."

Ms Hunter said 52 per cent of crashes occurred at night, and 43 per cent in wet weather, suggesting drivers were travelling too fast for road conditions.

But although the police intend monitoring the new limit, they are expected to "exercise discretion" until drivers get used to it, with prompting from electronic message signs over the next two weeks.

By Mathew Dearnaley Email Mathew