The judge said Mihinui used the card between November 2012 and January 2013 on eight occasions to withdraw amounts ranging from $20 to $700. She kept the money, which totalled $2180.
Judge Munro said to hide her offending, Mihinui collected the woman's mail and removed the bank statements.
"I hear you feel remorseful and so you should because you completely betrayed the victim's trust," the judge told Mihinui.
Mihinui's lawyer, Nicky Scott, said Mihinui had lost her possessions in a house fire prior to the offending and was unable to meet her student loan and car payments. Ms Scott said Mihinui was "extremely remorseful, very ashamed and willing to make reparation".
She said her client was on the unemployment benefit but would be able to repay the money at $20 per week.
Judge Munro said she acknowledged Mihinui had been having financial difficulties "but you know very well the answer to that is not to steal".
Rotorua Grey Power president Rosemary MacKenzie said this type of exploitation was far more common than many were prepared to admit - and was committed both by carers and family members. She said despite warnings about card security, if an elderly person couldn't get out to shop they had no choice but to trust someone to do it for them.
"You are between a rock and a hard place."
Mrs MacKenzie said she advised people where possible to give the carer/family member a certain amount of cash and ensure they got a till receipt.
The case comes ahead of World Elder Abuse Awareness Day on Saturday, which aims to draw attention to all forms of elderly abuse.