Joan Jessome, president of the Nova Scotia Government and General Employees Union, said Wednesday she expects the strike to last about 30 hours.
"When the legislation passes, we will go back to work," she said after a protest rally at the legislature.
The union represents 2,400 nurses who are demanding higher staffing levels to ensure patient safety. The Capital District Health Authority has said the demand for nurse-to-patient ratios won't work because it is too inflexible and there is no evidence it would improve safety.
The impact of a strike would stretch beyond the city because Halifax hospitals serve as regional health centres for the Maritimes.
"Every day that we don't have our nurses here is a concern for us, every hour that we don't have our nurses here," Capital Health CEO Chris Power told a news conference Wednesday. "So this is a huge concern for us and for the safety of our patients."
Power said the labour unrest has made for a tough work environment.
"This is a difficult time for everybody, not just for nurses who are poised to strike tomorrow, but all staff. Tensions are high."
The union and the health authority have agreed to maintain full staffing levels for emergency rooms and units offering dialysis, cancer care and intensive care. But Power said many other services won't have enough nurses to provide proper emergency care.
As an example, she cited the surgery unit for people with brain tumours and brain injuries, saying there won't be enough nurses Thursday to handle an emergency. Power said the same was true of the coronary care unit, which is full of ill patients but will have 70 per cent of it regular nursing staff.
The hospital that offers long-term care for veterans will have 66 per cent of its regular nursing staff on duty, she said.
"These people live there and require our nurses to care for them," Power said "We will use every effort to provide the best care we can ... but our patients and the provincial health-care system will be at risk."
Dr. Patrick Ward, the authority's chief of critical care, gave a more ominous warning.
"Is is possible that someone can be hurt?" he asked. "The answer to that would be yes."
Jessome said that up to 60 per cent of the union's members in Local 97 will be working Thursday.
On Wednesday, hospitals in the city handled only emergency cases and outpatient surgery. About two dozen more serious procedures were cancelled, even though the facilities were fully staffed.
Some patients have already been transferred to other health districts in the province.
Premier Stephen McNeil has said the new law is needed because there have been three labour disruptions in the health-care sector within seven months.
"There is no place in the country where staffing ratios are in any collective bargaining unit. Yet, that's what they're demanding here," McNeil said Wednesday outside the legislature. "We need to ensure for Nova Scotians there's a level of essential service ... if there's a labour disruption."
McNeil has said the law would not take away a union's right to strike because that option remains open once essential staffing levels have been negotiated. The nurses' union says such deals can take up to six months to reach, effectively undermining the leverage of a strike.
The government introduced its essential services legislation late Monday, prompting an illegal walkout the next day by a few hundred nurses. The health authority said the walkout forced the cancellation of dozens of surgeries.
The union later advised its members to obey a Nova Scotia Labour Board ruling that ordered them back to work.
The Liberal government's legislation would apply to nurses, paramedics, ambulance dispatchers, hospital employees and people who work in homes for seniors, youth and people with disabilities. In all, about 35,000 to 40,000 workers would be covered by the law.
The legislation would also allow parties to request conciliation or mediation to help negotiate an essential services agreement. If they can't agree, either party could apply to the Nova Scotia Labour Board.
Note to readers: This is a corrected story. A previous version said the essential services legislation included 911 operators.