The Cancer Society says patients won't make it to treatment unless an underfunded travel programme is reformed.
The National Travel Assistance Scheme was set up in 2005 and supports people who need to travel long distances or frequently for treatment.
A government review in 2018 found it was underfunded, too complicated and created inequities for those who need it most.
Cancer Society chief executive Rachael Hart said some people with cancer living in disadvantaged communities were already struggling to meet costs, and that would only get worse as the population aged.
"It's now vastly out of date. People can't get to treatment because it's just too costly and the scheme just hasn't kept up with inflation.
"The amounts that are paid to patients are not going up and it's actually stopping people from accessing treatment."
The health reforms would not meet goals if the next government didn't solve this issue, Hart said.
"People simply will not make it to treatment when the cost of travel is too high or when they can't access support easily, then they simply make decisions either not to go to treatment or to choose more extreme treatment options to shorten the amount of time that they might have to travel."
Reforming this scheme was their number one priority, she said.
The Cancer Society has released a manifesto seeking more investment in prevention, improvements to early detection and diagnosis, and better access to cancer care services that is closer to home.
It includes establishing bowel screening from 50 years for Māori and Pasifika people by the end of 2024, reforming alcohol laws to address alcohol harm and cancer, eliminating cervical cancer with vaccinations and free screening, and developing a national lung screening programme.