The Canadian Union of Public Employees says it was informed of the city's plans Monday night.
The union says the move has taken the 140 workers by surprise.
Mayor Randy Simms says the city wanted to put an end to the uncertainty that's been hanging over the municipality and control the timing of a labour disruption.
Workers rejected the city's most recent offer, which included a wage increase of about 12 per cent over four years.
They want a deal similar to one received by their counterparts in the neighbouring city of St. John's, which included a raise in the range of 18 per cent.
Household garbage collection will move to a biweekly collection schedule beginning Wednesday because of the lockout.
The city's community centres and swimming pool are closed to the public and recreational programs have been suspended.