Napier CBD's $5.2 million drainage upgrade has more than two years of digging ahead of it to lessen the flood risk, says Napier City Council infrastructure director Jon Kingsford.

"Typically when you design for a stormwater drainage network you design for a one-in-ten-year event, but the network we are installing is for a one-in-50-year event," he said.

"That makes everything bigger - bigger pipes take longer to lay."

He said the highest point in the CBD was the foredune system "which creates some real difficulties when it comes to drainage".

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Future street digging would only occur during "the quiet periods", outside of special events and holiday periods, and would be completed by the council's own team.

"Part of the challenge of doing this work is it is deep infrastructure - we have to go deep which has a lot of implications. Once you get below 1.5m, in terms of your trench, your health and safety requirements get really complex."

The programme also includes the renewal of sewer lines and working with other utilities.

So far stormwater pipelines have been laid in Hastings St, between Vautier and Emerson streets, and from the intersection of Hastings and Tennyson streets to the drain under the beach viewing platform.

He said streetside sumps would prevent debris reaching the beach.

"It won't deal with everything but it will deal with most stuff."

The end of the drain has a steel plate over it to stop gravel entering, from which some water can escape, but large amounts of water would escape via offset sidewalls. "There is a lot of storage in the bottom part of that pipe before it will tip over." Mr Kingsford said there was a fault when the first drain end-plate was mounted but lost somewhere on the beach. It was "pretty challenging" pouring concrete between high tides.

The upgrade has so far cost $3.8 million from a budget of $5.2 million.