Auckland City's most ambitious transport venture since Britomart will start business this morning when the first buses roll across a strengthened Grafton Bridge.

The renovated 99-year-old ferro-cement bridge will be part of the city council's $43 million Central Connector busway between Britomart and Newmarket, although it will start with only Link buses, because of congestion created by railway construction at the southern end of Park Rd until the end of next month.

Cars and trucks will be banned from the bridge between 7am and 7pm, when it will operate with dedicated bus lanes.

Trespassing will incur a $150 fine. Motorcyclists, cyclists, pedestrians and emergency vehicles will be allowed.

The bridge previously had a zero seismic rating, and city mayor John Banks once said it was lucky not to have ended up as a pile of rubble at the bottom of Grafton Gully.

But the elderly Auckland landmark has been strengthened to carry 44-tonne vehicles and to withstand an earthquake deemed unlikely to strike more than once in a thousand years.

It will be strong enough to carry a light-rail system, should that become necessary some day to increase its carrying capacity above 65,000 daily bus passengers.

The bridge's new strength is in contrast to that of an earlier cable structure across the gully, which Mr Banks said was so precarious police were occasionally posted at both sides to ensure drunken sports crowds did not cause it to collapse by jumping or stamping while crossing it.

Grafton residents who joined the mayor and transport officials at a reopening ceremony yesterday were safe in the fact that almost $7 million had been spent in a 12-month strengthening project which included bolstering concrete sections with carbon fibre straps and injecting epoxy resin into more than 800m of cracks.

But although 2800 faults were found in the bridge, project manager Ashley Cooper of Fletcher subsidiary Brian Perry Civil said most were minor and only two pieces of reinforcing steel had to replaced.

Bridge designer Will Pink, of Beca Infrastructure, who had to learn to abseil to inspect the structure 43m above the gully, said he remained in awe of the original builders and their use of experimental technology to construct what was at the time the world's largest ferro-cement bridge span.

The central arch takes up 98m of the 284m bridge, and was built on top of a mountain of imported timber form-work.

But despite the early engineering feat, Beca general manager Brent Meekan said the city council construction contract was so draconian that the Australian company which built the bridge became insolvent in the absence of provisions for any progress payments. "Nobody believed the bridge would actually take the weight it was designed to take [so] the contract said no payments until the bridge was finished."

Although the rest of the busway has been made wide enough to carry general traffic as well as buses, which will run on priority lanes on both sides of Symonds St and city-bound on Park Rd, the bridge's status as a Category A historical monument meant it could not be enlarged.

Mr Banks said that although the "visionary" early mayor Sir Arthur Myers wanted to build it twice as wide as the two lanes it now carries, the people of Auckland thought that would be too costly. The contract price was £31,610 ($4.6 million in today's terms), although the cost ended up at about £33,000 after the council had to complete the project because of the insolvency.

Fulton Hogan chief Bill Perry, whose company completed the other $37 million of work on the busway project, thanked neighbours and motorists for their patience and said meticulous planning and extensive communications had limited both disruption and complaints.