Nearly half a month's worth of rain has been recorded in one hour as thunderstorms passed through the region overnight.

The storm started with flashes of lightning appearing across the Tauranga sky around 10.30pm on Sunday.

At Mount Maunganui, the 20-30 seconds gap between the flashes was accompanied with quiet, no crash of thunder, wind or rain, just the distance sound of the sea.

This quickly changed and within 30 minutes the rain came on thick and fast.

Usually it passes over and through, chasing itself across the Tauranga Harbour from the Kaimai Range on towards the Pacific, but this rainfall was intense and not in a hurry to push on out to sea.

MetService issued a severe weather watch for the Coromandel Peninsula and Western Bay of Plenty on Sunday evening.

At 10.30pm, Coromandel Peninsula was upgraded to an Orange Heavy Rain Warning, while Western Bay of Plenty remained under a Heavy Rain Watch.

While many of us slept through it, firefighters received calls from about 10pm to 1.30am about flooded houses in Thames-Coromandel and Bay of Plenty.

A Fire and Emergency spokesperson says firefighters were called to seven flooded houses in Whangamatā, and 10 in Tauranga overnight.

Firefighters tried to pump water out of the houses, where possible.

They were also called to a tree down in Pukepoto in Thames-Coromandel at 1.22am.

Whangamatā chief fire officer Nigel Airey says a couple of the houses had water reach two-feet high in the basement.

Although damage was mostly garages and basements, water had also encroached on the homes of some out-of-towners.

There wasn't a lot firefighters could do to help, he says.

'But once its water damaged, it's damaged.”

They would return to the homes on Monday to evaluate and see if water could be pumped out.

Nigel says the main street of Whangamatā had been flooded, 'but the front raced through pretty quickly”.

'The water table is so high so any heavy rain, straight away it gets into buildings. Backyards are pretty full again.”

He says the rain was 'torrential” for about an hour and a half, but was constant all night.

Two fire truck loads of firefighters were out till about 2am, says Nigel.

On Monday morning, Waka Kotahi NZTA reported that State Highway 25 is closed between Hikuai and Whangamatā Road due to a washout.

MetService meteorologist Peter Little confirms that indeed, the Tauranga region received an extraordinary dumping of rain overnight.

'You guys had some pretty heavy rain last night. Our station at Tauranga Airport had 49mm of rain, which is a significant amount,” says Peter. 'That was in the hour up until midnight.”

'We also have a station in Golden Valley which is near to Waihi. That had 31mm in the hour up until 11pm last night.”

Putting this into context around what is ‘normal' rainfall for the area, Peter says at the Tauranga Airport site, the climate average rainfall for the month is 120mm.

'That's what you'd normally expect in the month of March. So having 49mm in one hour is like having nearly half the month's rainfall in one hour.

'We normally say if we have more than 25mm in an hour, we consider that to be torrential rain.”

Peter found it astounding that there weren't more reports of flooding around the western Bay of Plenty.

'That is a lot of rain, especially in an urban area, so I'm quite surprised there were so little reports of flooding. I would have certainly expected surface flooding on the road, because with that amount of rain, the drains aren't designed to carry that intensity of rainfall."