Auckland Transport this afternoon warned registered Hop card users of potential disruption between 10am and 3pm.
However, Ms Haskell said "full services will run until 11am."
That followed the company's receipt of a notice from the Auckland Tramways Union and the First Union of a three-hour strike from 11.15am, to allow a stopwork meeting to discuss differences with NZ Bus over pay and working hours.
The company has offered a wage rise of 0.75 per cent against a claim by the unions for 2.95 per cent, and is refusing to water down major rostering changes requiring most drivers to work rotating shifts.
Although the stopwork meeting was initially scheduled to run from 11am to 1pm in the Mt Eden War Memorial Hall, First Union organiser Rudd Hughes says it will be shortened to minimise disruption to passengers during the Christmas week.
It is now due to run from noon until 1.30pm.
The unions balloted their members over whether to take industrial action after the company refused to support the meeting as a paid stopwork.
One industry source told the Weekend Herald that 94 per cent of an undisclosed number of drivers voted "to take industrial action over rosters and snub their noses at the company's request to run skeleton crews".
But Mr Hughes said union leaders would strongly recommend that the drivers return to work from the meeting, pending more negotiations, which the company has scheduled for late January.
Although recent pay settlements at NZ Bus have been reached without major drama, Auckland commuters suffered serious disruption in 2010 when the company locked out its drivers for a week.
Anger over the roster changes, which the company introduced in July and which the unions are challenging before the Employment Relations Authority, is raising fear of another long-winded dispute while Auckland Transport is encouraging more people to catch buses in anticipation of major construction projects through the CBD next year.