Managed retreat needs to be seriously considered for homes around the rising Lake Rotomā, Rotorua's mayor says.

The lake is just a few millimetres away from overflowing onto State Highway 30, the lifeline between Rotorua and Kawerau.

Parts have already flooded. Lake Rotomā is also seriously threatening houses on its shore.

After months of rain, land around the Bay of Plenty is waterlogged, with NIWA's water soil map showing the ground is totally saturated.

NIWA has also revealed 11 locations across the country have already seen more than a year's worth of rain in the first six months of this year.

Rotorua mayor Tania Tapsell says the council is planning for the worst.

Rotorua mayor Tania Tapsell. Photo: Supplied.

"This is not like a river or the sea. Most of the time these lake levels do rise slowly but the problem is they don't go down.

"So we're not actually discussing evacuation, we're discussing something much more scary, and that's called managed retreat."

The council is yet to determine who could stay and who will have to leave, Tapsell says.

"We have set up teams between our council, Bay of Plenty Regional Council, local iwi and the Ministry for the Environment. We're looking at what we can do, not only in the short term to support the residents, but of course those big decisions on, well, can these homes stay where they are?"

Rotorua Lakes Community Board chairperson Phill Thomass told The Detail a number of houses near Lake Rotomā have already been flooded.

"A slowly developing disaster is I guess how I'd describe it, because water just keeps coming in and there's no way for it to go away, other than seepage and evaporation."

Tapsell says the consequences of the flooding are widespread.

"Unfortunately for some it does mean a long detour around the lake. This is impacting schools, for example. I know some students are only able to have half days so they can get in the bus and go the long way around to get back home."

Waka Kotahi says it's working closely with councils from Rotorua and Whakatāne to manage flooding on State Highway 30.

The transport agency says a short-term resilience improvement programme for State Highway 30 is underway, with rock armouring being added to the lakeside bank.

Lake Rotoehu, just west of Lake Rotomā, is also seriously affected by rising water levels.

Adrian Holmes, who has a two-storey holiday home at Ōtautū Bay, says flooding began in January.

"The house started flooding downstairs due to groundwater, not lake water. It was just pure saturation of the land and the water was coming up through every crack it could find in the concrete.

"Now we're facing the lake coming into the house, as well as the groundwater, so this is a new problem and the lake brings a lot of rubbish with it."

Holmes bought the place two years ago because he and his family wanted to be lakeside.

"Now we're in it," he says.

Like other property owners, Holmes has moved his belongings off the ground level and expects power to be cut off by floodwaters any day.

Nearby, the handful of permanent residents at Kennedy Bay have been cut off from the rest of the community because the road connecting them with State Highway 30 is underwater.

They include Julie Riggir, who now has to drive an hour to the shops, school, and social gatherings at the local hall - a trip that used to take five minutes.

It also means children in the settlement don't go to school and workers' 20-minute commutes now took up to two hours.

The road could stay closed for months, Rigger says.

Meanwhile, NIWA data shows the Bay of Plenty is not the only region dealing with unusually high rainfall.

Eleven locations around the country have recorded more than a year's worth of rain in just six months.

Kaikohe, in the Far North, has fared the worst - receiving more than 130 percent of its average annual rainfall - and it is only July.

Also making the list are Whangārei, Warkworth, Leigh, Whangaparāoa, Albany, Māngere, Tauranga, Gisborne, Tutira and Napier.

MetService forecaster Lisa Murray says climate change, a marine heatwave and La Niña are all partly to blame.

"All these things have combined along with a bit of being unlucky as well, because they could have gone a little way east, they could have gone westwards, but actually they came down over New Zealand. So we have the situation where we've had event after event after event. Some of those regions are very saturated even still."

However, Murray says the country can expect a change in the weather sometime soon.

"The systems we have coming through at the moment, and continuing really for much of July, are very fast moving systems. So although they will come in and bring some heavy rain, they will move through quickly."

MetService has forecast more rain for the east from Sunday.

- Finn Blackwell and Jordan Dunn/RNZ.