About 1000 St John Ambulance workers will be refusing to work at summer events until February.
It's the latest strike action being taken by members of the Ambulance Professionals First union, who are trying to negotiate a new collective employment agreement with Saint John.
Ambulance Professionals First spokesperson Lynette Blacklaws said "hundreds" of ambulance officers had spoken out online and on the job about the staff shortages.
"Yet St John has dismissed their concerns, even though union membership has grown by almost 200 members since the campaign to highlight these issues started."
The ban on working events, which follows other strike action in which ambulance staff refused to wear their uniforms, was expected to run until February.
The union said the action started today and union members would not staff the Pascoe New Zealand Tennis Championship at the ASB Tennis Centre.
St John's spokesperson Norma Lane said the strike would not have a large impact on events.
"We are committed to ensuring New Zealanders' holiday plans and summer events are not unduly disrupted by this action. It is disappointing the union is seeking to unnecessarily alarm the public."
She said St John had a flexible staffing model for its events service, which involved employees who belong to other unions, non-union employees, casual staff and volunteers.
She said the 2 percent pay rise sought by the union was not affordable for St John.
"St John received an increase in funding for labour costs of 0.4 percent for 2016/17 and is offering ambulance officers an increase of 1.25 percent, backdated to 1 July 2016 and rising to a minimum of 1.7 percent by 2018, plus additional leave."
Ambulance staff from four other unions reached a deal last month with St John Ambulance over a new collective employment agreement.
The events ban runs until 6 February 2017.