Hutt Valley Tranzit bus drivers have voted to protest what they call unlawful and dangerous shifts.
Wellington Tramways Union Secretary Kevin O'Sullivan said the industrial action was "a last resort".
"We have spent the last year and a half trying to talk to the regional council chair Chris Laidlaw, to try to get the council and Tranzit to sit down and have an adult conversation about drivers' terms and conditions and how to have a smooth transition of services between the old and new providers.
"They have refused.
"Instead we've had Tranzit running its operations out of makeshift depots and squeezing drivers with schedules that are unlawful and unsafe."
The action would consist of the 50 drivers cutting their daily hours to a standard eight-hour day with breaks, instead of the 14-hours some were rostered to do, Mr O'Sullivan said.
Tranzit managing director Paul Snelgrove said the industrial action was a "trumped up" move by the union.
He said company was prepared to discuss a possible collective agreement but it would not put up with scare-mongering or intimidation.
Mr Snelgrove said he was confident the company could handle the situation but that it was a shame the union seemed intent on punishing commuters.
The industrial action follows escalating tensions between the union and Tranzit, which culminated in both parties calling the police on each other.
Police were called at around 11am today after two members of Tramways Union entered the Tranzit bus depot in Avalon.
Mr Snelgrove said the two men were asked to leave the property but refused.
Police said they responded to a call that two men were trespassing at a property on Taita Drive and were verbally abusing staff.
However, once they arrived the situation had already been resolved and no arrests were made.
Yesterday, it was the union who contacted police, laying a complaint with the Commercial Vehicle Safety Team about driver hours.
Mr O'Sullivan said the rosters were unfair and a risk to public safety in some cases.
A police spokesperson confirmed the union laid a complaint yesterday and it was being investigated.
The breakdown in relations between the company and the union comes off the back of a major overhaul of the region's public transport network which came into effect on Sunday.
Greater Wellington Regional Council, which oversees Wellington's transport network, said it would not be intervening in the dispute.
Chairman Chris Laidlaw said the council was not the employer.
"If there is a breach of the requirements, the company will have to make sure it doesn't happen again."
Change of this scale and complexity was bound to bring with it "a degree of disruption and discomfort" for commuters, he said.
"There will be further discomfort still to come in the next week or so as the network settles in."