Heavy rain has begun drenching parts of Auckland just hours ahead of the region's borders reopening, setting cooped-up residents out into the country's wilderness. 

Niwa says tropical moisture combined with the cloud connection from the tropics to New Zealand has fuelled perfectly for thunderstorms - which will soon include strong winds and heavy rain - to smash the country over the past 24 hours. 

Parts of Feilding and Manawatu have been left under water, while a train derailed due to the main trunk line being flooded last night, sending its carriages into adjacent farmland. 

The MetService this afternoon said the rain which has now hit Auckland will likely remain persistent for the remainder of the day. 

Counties Energy and Vector are now warning Aucklanders of possible outages this evening due to high winds. 

Vector's General Manager Operations & Maintenance, Marko Simunac said speeds could reach more than 60km/h in places. 

"Following a period of rainfall over the past few days, the soil is still saturated and, together with strong winds this could unfortunately see an increased chance of trees or vegetation debris damaging power lines. 

"I want to reassure people we have crews available to respond and have made preparations to be as ready as we can ahead of any impact this weather might bring. 

However, Counties Energy were warning the high winds will hit gust between 70km/h and 90km/h between 8pm and midnight. 

They will spark up again about 1am tomorrow until around 5am and gust up to 100km/h. 

"These winds could cause power outages so please do your storm prep," it wrote on its Facebook page. 

One of the carriages remains upright after a derailment south of Hunterville last night. Photo / Bevan Conley 

Waka Kotahi NZ Transport Agency is also actively monitoring Cyclone Ruby as it tracks towards New Zealand. 

Jacqui Hori-Hoult, regional manager operations and maintenance, said the weather event had the potential to impact the state highway network with heavy rain warnings already in place for parts of the North Island. 

She urged motorists to travel with care especially given the reopening and removal of the Auckland borders tonight. 

"On the night of December 14 southbound traffic can expect closures in place from 10pm to 1am as the boundary is removed while northbound traffic can expect stop/go traffic management in place until midnight, with the boundary removed by 1am. 

"In the days following we know that people across the country will be looking forward to reconnecting with friends and whānau ahead of the holiday period and we are expecting the roads to be very busy." 

With this in mind, Waka Kotahi has decided to pause all planned maintenance and capital works involving full road closures on SH1 in Northland, Auckland and Waikato for a period of five days from Wednesday, December 15 to 8pm Sunday, December 19.