St John Ambulance says concerns about the level of coverage and training provided in greater Auckland are unfounded and there are never fewer than 38 vechiles operating.
The National Distribution Union, which represents paramedics, says cost-cutting means at times, a single ambulance is covering large areas such as Waitakere or North Shore.
The union says the financial constraints are caused in part by the recession and a drop in donations that would normally help top up government funding. It believes lives may be at risk.
The union's ambulance sector coordinator, Craig Page, says issues over shifts can affect the time it takes to respond to emergencies.
He says there are more cuts affecting team managers, who often provide life support back-up, and first-response coverage in more mobile vehicles.
St John says though there are financial pressures, it is business as usual. It says on Thursday, there were 40 ambulances operating in greater Auckland alone.
Northern regional operations manager Gary Salmon says there are no reports of drops in reponse times and there are never fewer than 38 vehicles operating throughout greater Auckland.
"We come to work every day to save lives - that's our business. And there's nothing that we would do as a management or team to deliberately put anybody's life at risk," he says.
"Our patient safety is paramount, and we constantly focus on that requirement."
However, Mr Salmon says financial pressures mean St John is handling sick leave more stringently.