Both the Whanganui and Whangaehu rivers are expected to peak later, after heavy rain - which has battered much of the central North Island for the past 24 hours - caused the rivers to swell.
Horizons Regional Council has activated its emergency operations centre, along with Wanganui, Rangitikei and Ruapehu District Councils.
Some residents have already been cut off in Ohakune and Ruapehu as lashing rain saw rising water levels and slips, which forced the closure of State Highway 4 between Wanganui and Raetihi.
Rising river levels may also close State Highway 3 at Whangaehu, Horizons incident controller Ged Shirley said. However, his team will continue to monitor the situation and liaise with the Transport Agency.
"There is potential for the road to be closed this evening,'' he said.
The Whanganui River is forecast to peak at the Pipiriki monitoring site at 6.30pm, reaching a level of at least 15 metres, with the lower river expected to peak at 9 metres around 11pm tonight, said Horizons Regional Council.
It warned that evacuations are possible, and potentially-affected residents are being contacted by Wanganui district Council.
"We are working with Wanganui services to monitor the situation and our barrier team are on standby in preparation to deploy the temporary flood barrier at Balgownie,'' Mr Shirley said.
Staff are also monitoring water levels on the Whangaehu River, which is expected to peak at 10 metres around 9pm this evening. With evacuations also possible in this area, Rangitikei District Council are contacting potentially-affected residents.
The Waihenga Bridge to Martinborough may also be forced to close due to high river levels, said Mark Allingham, general manager of infrastructure and services for South Wairarapa District Council.
The region was badly hit with power outages during last night's storm, and residents in Greytown were forced to go for a period without water this morning, he said. The area was running on reserve water supplies, but without electricity the emergency bores could not work.
"We got the emergency bores going just as we were running out, so we'd used all our reserves but we still had some water left in the pipes,'' Mr Allingham said.
In other areas of the region, the Makino Stream and Rangitikei River are also being closely monitored with no cause for concern at this stage.
Around half a dozen Ohakune residents living on the Mangawhero River Terrace extension have already been forced to evacuate as the river continued to rise.
Ruapehu District Council has established a welfare centre at the Ohakune Primary School hall.
Council chief executive and Civil Defence controller Peter Till advised any residents who feel that they are at risk to self-evacuate.
Meanwhile, wild winds are have been lashing Auckland city today, with shattered windows, lifted roofs and runaway trampolines.
The wind caused flight disruptions at Auckland Airport in the late morning and early afternoon, with both domestic jet and Air New Zealand Link services delayed and several flights diverted to alternative ports.
One international service NZ700 from Sydney was also diverted, to Wellington, with passengers expected to arrive in Auckland about 4.30pm.
A gust blew out glass panels from an overhead covering at a central city hotel.
Three glass panels were shattered by an upward gust of wind at the Rydges Hotel on Federal Street.
No one was injured in the incident which took place before midday.
The Fire Service was called to make it safe, smashing out and removing the damaged panes.
Sally Stoikos, who was visiting Auckland with her family from Melbourne, said they saw the firefighters using hammers and axes to remove the broken glass.
Pedestrians in the central city were seen clinging to poles as the wind hit.
Power has been knocked out in Whitford, near Flatbush, and there are reports the roof on the Countdown in Papkura on Great South Rd is lifting.
Trees have fallen on a houses in Avondale and bricks have been ripped off a two-storey house in Parnell.
Beachhaven resident Jackie Gray said a trampoline from across the street flew over the fence and into her front yard.
"It would have come inside. It was just hanging on the bush, thank God my hedge was big enough.
"The neighbour and police came, and we just held it down.''
The trampoline was now being weighted down by ``bits of concrete''. It was also tied down, she said.
"I was scared. Bits of metal - the legs were metal - and all over the road. It basically would have come through the window.
Mrs Gray said once the Fire Service arrived, they helped secure her neighbour's garage roof in place which had been lifting off in the wind.
Next door, a tree about 8m tall fell onto the roof of a home.
Mrs Gray husband Stu, a local drainlayer, was helping property owner Julie Chessum clean up.
"It could have been a lot worse,'' he said. "The local community constable came down and he actually took the main parts down and we followed up quickly.''
Mrs Chessum said her tenant, who wasn't home at the time, would be surprised to have "a lot more light" streaming into her home.
"I'm just glad no one was hurt and my house in not hurt either.''
Power lines have snapped on Mangere's Tennessee Ave.
Mangere Fire Station officer Roy Harris said a "weather bomb or wind bomb'' had caused the lines to snap.
"We've got multiple power lines down through here.
"A really big gust of wind has just broken [the lines].
"Just on the way as we were coming over here there were calls to fences down, someone was looking after a roof off, trampolines have been flying around.
"It's just really big gusts of wind that just suddenly increase that force on things until it bursts.''
Mr Harris said some houses had lost power as a result of the downed lines.
"We're still waiting for the power authority to come and isolate the power and make it safe.''
The downed lines had been arcing but that had ceased after the relays tripped, he said.
Fullers said that Northcote Point was now closed for ferry services until further notice.
WeatherWatch.co.nz said winds were reaching gale force at times with gusts to 90km/h.
Winds of this speed are mostly below damaging threshold but wind tunnels such as in the CBD between buildings, and parts of exposed suburbs could see minor damage, especially branches breaking and powerlines being broken.
The warm wind direction had already pushed Auckland's high up to 21C, well above what both main forecasters were predicting.
"This is the rough weather that was expected to arrive around dawn, but is running a few hours late'' said head weather analyst Philip Duncan.
"This afternoon in Auckland winds will change from gale nor'westers to blustery sou'westers''.
The worst of the winds was at 11.30am, with gale force northerlies gusting near 100km/h. By 12pm winds were just below gale threshold.
Around 3000 homes are without power in the central and lower North Island as stormy weather continues to hamper efforts to fix electricity lines.
Nearly 20,000 properties in Wairarapa, Manawatu, Wanganui and South Taranaki have had power cuts since severe weather hit the country yesterday morning.
Connections had been restored to all but 3000 customers, Powerco said.
More than 150 field staff had been sent out to restore electricity but difficult conditions had hindered work.
Powerco network operations manager Phil Marsh said new faults were being found as repairs were being made.
He blamed trees for causing most of the damage.
"Many of the power cuts could have been avoided if people had contacted suitably qualified tree trimmers to have their trees cut or trimmed before they caused a problem,'' he said.
Mr Marsh said "good progress'' had been made in restoring power, with the majority of south Taranaki properties expected to be reconnected by tonight.
However, he said areas which suffered the most damage, such as Wairarapa, may still be without electricity for a second night.
