The Government needs to put an extra $700 million into health in the Budget just to maintain services at current levels, the Greens say.
Health spokesman Kevin Hague is citing an analysis by the Council of Trade Unions and the Association of Salaried Medical Specialists.
He says the analysis takes into account growing costs and population pressures, as well as the impact of an ageing population.
"The Government has failed to fund health properly and there will be real life consequences for all New Zealanders if National doesn't stump up with a massive health investment in Thursday's Budget," he said.
"The health budget is under continuing and severe pressure, district health boards are having to perform miraculous feats to ensure that New Zealanders' healthcare services are maintaining and key services such as mental health are falling through the cracks."
Last year's Budget gave health an extra $400M, similar to previous levels.