In Wairoa, concrete floats.
Heavy rain has caused delays to the completion of a 4.2-tonne concrete drain in Kitchener St, after the newly laid pipe floated out of position on top of the floodwaters.
Wairoa District Mayor Craig Little said it was something of an engineering miracle, and couldn't have been predicted.
About 70mm fell in the northern Hawke's Bay town over 24 hours on Wednesday, with a peak of 27mm within an hour.
"The heavy rain caused the 4.2-tonne culvert, the size of a couple of big cars, to start floating. You have got to keep water out of the culverts during installation but the engineers could not have predicted this," Little said.
"The pressure of the water made them float and rise."
Little said contractor Fulton Hogan was insured and the engineers were working on the issue.
"The engineers are all over it. It's just more frustration for the contractors, as it would delay installation by a couple of weeks."
The culverts will need to be removed and relaid.
A Wairoa District Council spokesperson wrote on Facebook that its engineering team had "never seen anything like it before".
The uplift pressure of the rain pooling at the bottom of the drain compromised the stability of the culverts causing them to float, in a freak incident.
Last year about 245m of concrete culverting was laid in Kitchener St as part of a Wairoa District Council plan to future-proof the town's stormwater network.