Auckland's harbour bridge turns 50 on Saturday. This week, the Herald looks at how it has changed the city.

Today, Mathew Dearnaley goes back to the beginning.

Before the mid-20th century, the North Shore was a sleepy backwater of seaside settlements, usually stirred to life only at weekends and public holidays by Aucklanders cramming ferries to invade its beaches and attractions.

But in the postwar boom, Auckland began looking hungrily across the Waitemata Harbour for development space to counter the urban sprawl chewing through agricultural land to the south.

Although about 50,000 people had made the Shore their home by the mid-1950s - compared with more than 220,000 now - its growth rate was still only half Auckland's average, hampered by the harbour divide.

It offered few jobs, meaning arduous daily trips across the water on the Devonport Steam Ferry Co's 19th century coal-burning vessels.

For those with cars, the alternative was a 50km drive via Riverhead.

But developers were drawing up expansion plans, confident time was nearing when pressure for a bridge or tunnel would triumph, despite the failure of proposals going back to 1860.

That was when engineer Fred Bell, commissioned by North Shore farmers wanting to herd their pigs and cattle to market, designed a drawbridge on floating pontoons along an approximation of today's crossing route.

Although it was to be paid for from tolls - a concept to be adopted on the route 99 years later - the scheme foundered over a prohibitive estimate of almost £16,000, or $1.6 million in today's inflation-adjusted terms.

After other schemes came and went, a royal commission in 1946 recommended a four-lane bridge.

But further investigations raised fears the commission had underestimated population growth, and six traffic lanes might be needed, prompting the Ministry of Works to opt for a five-lane compromise.

A harbour bridge authority headed by Auckland Mayor Sir John Allum was appointed in 1951 to raise public loans and call tenders for the plan, which also included two footpaths and a network of approach roads.

But the struggle was far from over. Prime Minister Sidney Holland shelved the project in 1953 in fright at the lowest bid of just over £8.1 million (equal to $396 million now) from a consortium of British engineering giants Cleveland Bridge and Dorman Long.

But after fierce lobbying from the authority, the Government said it would allow the bridge within a budget of £5 million ($245 million).

That produced the "austerity" bridge, with four lanes and its proposed footpaths dropped.

The authority even axed the main proposed approach roads, along St Mary's Bay and Shoal Bay. But the alternatives of Curran St and Queen St through Northcote whipped up enough of a public outcry to force the Government to reinstate the original network.

That pushed the cost to £7,515,840 ($294 million). Not even Sir John could rescue the fifth traffic lane, and pedestrians and cyclists have been battling to this day for the footpaths.

Loss of the fifth lane meant the bridge authority could not use tidal flow traffic management to control demand which exceeded expectations from opening day on May 30, 1959.

Far above the royal commission's prediction the bridge would be carrying three million vehicles annually by 1965; the volume exceeded 10 million that year, and was approaching 15 million when the four clip-on lanes were added in 1969.