How the Sydney Harbour Bridge could have looked

Ms Turnbull, who spent a year working on the project, said The Bridge series is a way of giving people access to the State Library's vast collection of information and imagery related to Sydney's most iconic landmark.

"I think people don't know about the history of what we could have had, I also think they don't know about the battles and how long it took, and the political struggles," Ms Turnbull said.

Before the bridge

By the time it was completed in 1932, Sydney desperately needed a bridge to connect the north and south of the city.

Ferries were the only way to transport people and goods across the harbour - by 1890, more than five million passengers were crossing the harbour each year.

"The ferry organisations were not very keen on any kind of harbour crossing because they had a monopoly of getting goods and people across the harbour," Ms Turnbull said.

But as Sydney expanded, it became obvious the city needed a bridge and the first design was submitted in 1840. Between 1900 and 1924, four public design competitions were held.

One Harbour, many options

Ms Turnbull, who calls herself a pontist - or a "bridge enthusiast" - said there were some "fantastical" designs submitted over the years.

"Then there was an idea for a pontoon bridge that would swing around, and then the other one that we've got is the 1840 design for a floating bridge, and I don't know how that would have worked frankly," she said.

There were numerous designs for cantilevered bridges, as well as a flat iron bridge designed by Peter Henderson in 1857 that would have impeded shipping, and even bills to create a harbour tunnel that were put to parliament in the 1890s.

"I don’t know how they would have dug the tunnels then," the curator said.

"It was even mooted to fill in the harbour, which I really don't understand how that could have worked anyway."

One of the more outlandish plans was a 1922 design by Ernest Stowe for a three-pronged bridge joining Millers Point, Balls Head and Balmain.

"Apparently there would be a lift system in the central pylon on Goat Island, where the cars or trucks - or carts as it was - would go in, and have to be moved up to go to the next layer to go out," Ms Turnbull said.

"I like the idea of what could have been, it's a fantastical approach to design."

Despite Norman Selfe winning a government competition for a cantilevered bridge design, it was a bow-shaped bridge designed by John Bradfield that won politicians over.

In 1924, more than 80 years after the first design was submitted, the Harbour Bridge Act was carried by NSW parliament.

Ms Turnbull said the third instalment of the State Library's series explores the sheer effort it took to ensure Sydney got the bridge it deserved.

"Bradfield, he sounds like he was very driven," Ms Turnbull said.

"The [bridge builders] who were interviewed speak highly of Bradfield, one of them said, 'he knew what he wanted, and he wanted it yesterday'."

See the State Library's interactive 3D models of various harbour bridge designs and their full series on the bridge, which is being released weekly, on the State Library website.