Napier City Council has urged residents to "let it mellow" and to avoid releasing wastewater, or else sewage will have to be discharged into a local estuary.
Very heavy rain has fallen in the region overnight, up to 100mm in some places, with a further 64mm forecast for today, the council said.
MetService currently has a heavy rain warning in place for Hawke's Bay, valid from 9am this morning until 6am this evening.
The rain has overwhelmed the city's wastewater systems, and the council has warned that if wastewater flows cannot be reduced, sewage will need to be discharged into the Ahuriri estuary.
"We're asking all of Napier to please keep their household wastewater load as low as possible for the next 24 hours," the council wrote.
"That means letting your yellow mellow (eeek!), short showers and not running washing machine or dishwasher cycles.
"If an emergency discharge is carried out, this should prevent wastewater backing up and overflowing into city streets and properties."
Surface flooding has taken place in several parts of the region this morning.
The New Zealand Transport Agency has issued a general warning for motorists in the region, saying caution is advised, and some minor slips have been reported on highways.
According to Hawke's Bay Regional Council's river level monitoring stations, the rain over the past two days has raised river levels in the region significantly, including raising the level of the Esk river by almost a metre.
Napier was forced to discharge treated sewage into the estuary in September last year after a similar level of rain caused sewage to begin to come out of manhole covers.
Sewage and stormwater was also discharged into the estuary following ex-Cyclone Debbie in 2017.
Former mayor Wayne Jack said last year that Napier's ageing wastewater network suffers from considerable "infiltration" by stormwater.