The big hole on the old harbour bridge causeway is called 'seismic enhancements” by engineers and is bringing the older part of the harbour link up to the same earthquake standard as the rest of the link.
The original harbour bridge causeway is sand. Two rows of rock were stacked across the harbour with sand pumped into the middle.

The shaking from a major earthquake would liquefy the sand, severing the harbour link, says project manager Kevin Reid.
They are going to drive 12-15 metre piles into the harbour bed along the path of the roadway, and sandwich layers of sand and geotextile material on top of them.
The piles will support a raft like structure intended to hold together in a major earthquake.