Workers are being encouraged to sign up because living on the pension alone can be difficult.
National media reports suggest some pensioners are also adding water to their milk so it lasts longer.
Mrs MacKenzie said high petrol prices and rising council rates were only making things harder. "If superannuation is your sole income, then you are struggling.
"It's fine if you just want to exist, but when you've worked 40 or 50 years, you kind of expect a little bit better in your older years."
Retirement Commissioner Diana Crossan says living on a superannuation allowance is tough. "People that work should understand that unless they put savings aside themselves they could end up living off New Zealand Super alone and that's really tough.
"It means fish and chips are a treat - and if you don't want life where fish and chips are a treat then you should start doing something early."
The KiwiSaver scheme was an easy and convenient way for employees to save for the later years, Ms Crossan said.
At the end of August more than two million New Zealanders had signed up with more than $12 billion now invested.
Individual workers have saved over $5 billion since the scheme's 2007 introduction. Government contributions total $4.88 billion and employers' contributions total more than $2.7 billion.
Ms Crossan said planning for retirement was about weighing up current options with future pay-offs, such as those associated with the KiwiSaver scheme.
"There are times in your life when you have to make these decisions about how you manage your money differently.
"It might be worth going into debt to get an education, because your education will give you a better income in the future.
"It might be also worth going into debt to buy a house and get a mortgage ... because owning your own home is part of your retirement savings."
About 8 per cent of New Zealanders aged over 65 lived in hardship, of whom many were renting their homes, she said.
Other figures show about 250,000 people have opted out of KiwiSaver since it was introduced and just over 5000 people have turned 65 and withdrawn funds for retirement.
More than 80,000 others had taken contribution holidays at the beginning of September. A contribution holiday can be taken by someone who has been on the scheme for 12 months and wants to take a break from saving.
KIWISAVER
Bay of Plenty savers: 115,833 participants
Total savers nationally: 2,002,840 participants
65-plus withdrawals: Just over 5000
Number of people on a contribution holiday: 86,468 at September
- Source: Inland Revenue