If you are unable to work due to the big C, you and/or your partner (if you have one) may be eligible to receive a Sickness Benefit or Invalids Benefit.
Regardless of whether you are working or not you can apply to receive help with disability costs such as prescriptions, transport to hospital, GP visits, utility bills etc. through the Disability Allowance. There is also an Accommodation Supplement you can apply for to help with mortgage and rental costs.
You can find out more at www.workandincome.govt.nz
Be warned, you will need to make an appointment to see someone face to face and there will be lots of form filling (your GP also has to sign and stamp) and documentation to take, but they are usually very lovely people who are a great help.
Working for Families
Depending upon your circumstances, and if you have children, you may be eligible for a Working for Families tax credit payment from IRD. A handy calculator on the site will give you a heads up if this is the case.
You can find out more at http://www.workingforfamilies.govt.nz
Its straightforward to apply and you can even do it online from your lounge.
Ministry of Health
Similar to the Harry Potter Ministry of Magic, this one provides funding for magically expensive and necessary items on the cancer journey like wigs and foobs (or breast prosthesis to use the correct terminology).
Details of funding are available here:
Your Oncologist or Breast Care Nurse will give you the appropriate signed forms to get this all underway.