Looking at a submarine cable map such as submarinecablemap.com can be a little misleading: One might think that a cable is like a road, with turn-offs and branches and intersections, and that when there are two or more cables reaching a particular coast or island, it necessarily creates choice and redundancy the moment a cable lands.
Well, not quite so. For one, a cable isn't a cable. Some cables carry multiple fibre pairs, which may be owned by or leased to different operators. Some of these might branch off at an intersection ("branching unit" or simply "BU" in Expertese) coming from one end, and their signal may not continue down the cable.
A well-known example here is the Tonga Domestic Cable Extension, which connects the capital island of Tongatapu to the two other major population centres of Vava'u and Ha'apai.The fibre pair that runs from Tongatapu to Ha'apai branches off at the BU towards Ha'apai's main town of Pangai, whereas the other fibre pair carries on to Vava'u. The cable got severed several times: First in a human-caused damage incident in 2019, then again after the Hunga-Tonga Hunga Ha'apai eruption in 2022, and again twice in 2024 after slips off the volcano's flank, in all cases between Tongatapu and the BU. Yet this also meant no connectivity between Vava'u and Ha'apai: Terminal equipment cards cost tens of thousands of dollars, and so there was simply no light path between the two islands - all the traffic between the two island groups travelled via Tongatapu. The map doesn't tell us.
Likewise, more modern cables now use fewer fibre pairs and instead rely on wavelength division multiplexing. Simply put, they use different colours of light for different destinations. A modern BU houses a prism that selects a particular wavelength (colour) to be dropped off towards a branch, and that wavelength does not continue on the trunk fibre. Read: The branch also has connectivity to one end of the trunk cable only. Again, the map doesn't tell us.
Operators may also pool their traffic on one wavelength on one fibre pair, relying on routers to multiplex and de-multiplex the traffic between their various networks and the cable. Furthermore, operators may load balance between different fibre pairs and cable systems, or may or may not have failover arrangements in place. Last but not least, just because two or more cables land in a particular area does not mean that they interconnect, let alone for all operators and all traffic. Some cables may not even connect at all - sometimes cables land years before the on-shore infrastructure is ready.
In places like New Zealand, VULGEO's main initial target, having multiple cables land means that traffic to and from different parts of the Internet arrives and departs via different cable systems. Which systems exactly for a particular web or mail server? This depends not only on where that server is, in whose network it lives, but also on who accesses it from which network in which location.
So when a cable fails, its impact on connectivity depends on a multitude of factors.