MetService says a new rain radar will get around the mountainous terrain and known blackspots in Nelson-Tasman, but its exact location is yet to be decided.
The government said work would begin immediately to procure a new radar for the top of the region - which has endured repeated floods caused by heavy rain - with the aim to have it up and running by the start of 2027.
A new radar would cost MetService up to $5 million, with operating costs of around $800,000 a year.
MetService general manager of observing systems Kevin Alder told Morning Report the Wellington radar that was being used for the top of the South Island - although geographically close - was imperfect.
"Its coverage over Nelson is blocked in the lower elevations by the terrain of the Richmond Ranges which are around 1500 metres, 5000 feet high.
"And so, we really can't see into Tasman Bay and the surrounding areas."
He said a radar on the ground in the area would solve that problem, and MetService would work with the district councils to secure "the best possible site" for it.
While the initial weather warning would be the same, it added in the detail once the weather system hit, he said.
"It gives much better information to emergency managers as the event unfolds and particularly the timing of when an event is going to end.
"We can track the rainfall, the severe weather in real time with the radar, and really help with those immediate decisions during the event."
Alder said the complex technology and associated infrastructure accounted for the multi-million dollar price tag.
"We normally have to put them on a tower and all the supporting infrastructure for it, with communication and data links - it all adds up."
He said although weather systems could be monitored from space, ground-based radar were the best detection method for an area.
"You'd have to have a lot of satellites in space to cover New Zealand."
The announcement at the weekend came after years of campaigning, with the region considered a blank spot in the national radar coverage.
Associate Transport Minister James Meager said the community had been asking for a radar, and its installation would not only give locals "peace of mind", but would help them better prepare for future disasters.
Emergency Management and Recovery Minister Mark Mitchell said Nelson-Tasman had been in five states of emergency since 2011, and a new weather radar would help emergency managers better monitor rainfall and flooding risk - "reducing the risk of loss of life and property".