The Auckland Harbour Bridge and other major bridges undergo a general inspection every two years and more detailed inspections every six years, says the New Zealand Transport Agency.

Responding to the deadly bridge collapse in the Italian port city of Genoa, the agency said some bridges have even more rigorous inspection programmes designed to identify changes to their condition.

While the cause of the Morandi bridge collapse in Genoa yesterday that killed at least 39 people is not yet known, there has been widespread debate over the state of the bridge and Italy's infrastructure in general in recent years.

It comes amid growing fury over the collapse, after it was revealed there were numerous ominous warnings over problems with the structure.

The highway bridge, in the northern Italian city, crashed to the ground without warning on Tuesday, with almost 40 cars and trucks falling 45 metres with it.

An NZTA spokesman said given the strategic importance and iconic contribution to Auckland's skyline, the Auckland Harbour Bridge has a a team of engineering specialists that undertake a wide range of activities to ensure the safe and reliable operation of the bridge.

"These activities are regularly tested and challenged to ensure they are consistent with international best practice for significant bridges and appropriate for the safeguarding of the Auckland Harbour Bridge," the spokesman said.

After engineers warned in 2007 of a potential for "catastrophic failure" in a worst-case scenario, 920 tonnes of extra steel was bolted and welded onto the clip-ons to extend the life of the bridge.

At the time, restrictions were placed on heavy vehicles using the clip-ons to reduce vibration and help keep the bridge stable during welding of steel inside the box girders.

Today, the clip-on lanes are open to 50-tonne maximum permitted heavy vehicles and heavier vehicles can only use the truss bridge.

Morning fog shrouds Auckland Harbour Bridge. 10 May 2018 New Zealand Herald photograph by Michael Craig

In 1996, the agency began a seismic screening programme to identify existing bridges that may sustain damage in an earthquake. Following this, strategic bridges, such as Wellington's Thorndon Overbridge and Auckland's Harbour Bridge, were upgraded.

The agency said it had completed the upgrading of bridges on high-importance routes needing minor upgrades and is completing the remaining upgrade of low-risk work on an annual basis.

It is progressing with detailed assessments and upgrade works for the highest-priority bridges that require significant upgrading.