Agree it's a rare event that doesn't need to be designed for,...

Agree it's a rare event that doesn't need to be designed for, but pipe culverts have the least capacity compared to box culverts and bridges and HDPE pipes are less durable than concrete one. On that basis, it is quite possible they are being used just to get the road reopened as quick as possible.

I've no idea of the catchment area but that amount of water would indicate there is a reasonable upstream catchment. The waterway structures under a main highway that is of national importance should be designed for the higher flows from rare events than the rest of the road network, ideally the 1 in 100 year event to minimise disruption to the community. Whilst the road may not be at that standard now, you would hope that if the existing structures are damaged and the government needs to rebuild, they would design the new structures to the 100 year capacity. Depending on the magnitude of the 100 year design flow, his could be achieved via multi cell box culverts or a bridge or even a causeway if it is designed to take the flows but not be damaged and can be reopened once the floodwaters recede.

Anyway, first steps are to get the roads reopened asap so the community and industry can use again, then hopefully the government are able to construct more substantial and durable structures for better serviceability post flooding.