The design presented by Waka Kotahi is probably the most carbon intensive way of creating a pedestrian bridge. Each 600mm wide column foundation is 20m to a solid base on the underlying gravel. There are at least 20 of them - so some 520m3 of concrete and steel foundations - that's ~1200 tonnes of concrete and steel. And then there are the columns above ground, and the pre-cast steel reinforced ramps themselves. We're likely talking another ~2500 tonnes. 3700 tonnes of carbon emitting concrete for no good reason.
And then the span - it looks like a 1920's steel rail bridge - and steel alone is also carbon intensive - though less so than concrete.

But even if there was magic no-carbon concrete there are other problems with the ramp design.
The concrete structure will be a graffiti magnet, and the undercroft will be a nasty place that will need to be fenced off to prevent night sleepers. There will have to be a limit on the type of vegetation as any tall bushes or trees will reduce sight lines and make the area under and around the spirals dark and forbidding. And plants do not grow under the concrete ramps - so they will be nasty places that will accumulate rubbish.
The design is extremely poorly thought out, with high environmental impact. It is way overbuilt and unnecessary. And if you look at any motorway overpass this is not the way they are built... and it's for a reason. It's not a sensible way to build.

So if there must be a bridge crossing then it has to have a better urban and environmental design.
Berms and small hillocks could easily form the ramps (as they do for motorway overpasses) - which could have paths that are properly separated between pedestrians and those on bikes as there is no expensive pre-cast structure. It is easy to form two routes when the added expense disappears. Just lay the pavement along a different route around the berm and slopes.

The berms can be made from gravel and solid landfill - saving the embedded carbon that is in concrete and steel. This material is lower cost and uses much less expensive labour. There also a lot less coordination required - all those pre-cast ramps have to align just so - it will be faster, easier, and cheaper... and much more forgiving of mistakes... I'm looking at you Parakiore Sports Complex.

This landscape element can then be fully planted with trees, and bushes as required - there is no undercroft that would create a dangerous feeling space. And a basketball court could still play a part.

The berms would also serve to reduce noise traveling down Collins and Simeon Streets - the earth will absorb and reflect noise. It would also help disperse and reduce air pollution, vegetation will help in this too.

There is no advantage to concrete columns and pre-cast ramps other than that is what engineers can design. So here the saying 'If you give an engineer a hammer all they see is nails' comes to the fore.

Waka Kotahi should think out of the box here and set out the requirement to minimise carbon intensive construction for this project from the outset - and not rely on subsequent contractors and consultants to risk proposing an alternative. The contractors will build what Waka Kotahi have shown to be their intended scope - concrete ramps. Change the scope now - not later.

The location of the ramps would have to be modified to reduce the need for retaining walls by moving them away from adjacent properties - but the current ramps are located because they need to be pre-cast concrete repeatable forms. For the concrete ramps the closeness to boundaries remains a problem - overlooking and the throwing of items into neighbouring properties will likely require high opaque fences on the concrete ramps. Every time you consider the implications of the Waka Kotahi design more problems emerge!

The bridge span across the road is indicative only - however Waka Kotahi need to require that a low carbon structure be built. There are many many mass timber pedestrian bridges that can be referenced from around the world. I suggest this be a requirement too. The forms of these bridges can be spectacular - but with the savings in construction costs by removing concrete ramps the budget for a truly beautiful gateway bridge would then be well within budget.

We really don't need or want a bridge (or underpass) here - we need a properly designed level crossing. But if we must have a pedestrian bridge then let's at least have one fit for the 21st century - and not some carbon spewing concrete carbuncle that will blight our neighbourhood for decades.