Health needs an additional $555 million in the budget just to keep up with population growth, ageing, medical advances and staff costs, the Council of Trade Unions says.
"This is only what is needed to stand still," said CTU policy director and economist Bill Rosenberg.
"If we want improvements in the health system or to address existing problems such as persistent deficits in district health boards and loss of services, further increases in funding are required over and above this."
The union said $555m would be more than half of the $1.1 billion the government would allocate to new spending in Thursday's budget.
"It is well above the 40 percent of the operating allowance (equivalent to $440 million) which is the assumed allocation for Health in Treasury forecasts such as the December Half-Yearly Economic and Fiscal Update," Mr Rosenberg said.
"The CTU and its affiliated health sector unions have made this estimate so that we and the public can judge whether the Health vote announced in the Budget on 20 May provides for improved health services, or is really a cut in those services, or a stand still."
Executive dgGovernment to listen and said the assessment had not even added the cost of meeting government goals such increasing the role of health professionals in decision-making, addressing workforce shortages and more surgery.
"In last year's budget the government increased funding to DHBs by around $750 million. In this year's budget the rate of increase is expected to be dramatically slashed by around half this amount. There is a serious risk that using such a blunt fiscal instrument will force DHBs to adopt 'shock therapy' measures with the victims being patients."
Mr Powell said DHB deficits did need to be bought under control but they did amount to less than 2 percent of their funding.
Health Minister Tony Ryall said the unions would have to wait to see what was in the budget.
"All will be revealed on Thursday. The government continues to invest strongly in improving frontline health services, like record elective surgery, faster cancer treatment, faster emergency departments and cardiac surgery.
"We are moving resources around to provide the improved services patients want."