The twins (22) work at Moyle's Fresh Choice, in Green Island, and give plasma every two weeks.
''It gives us an opportunity to help people out,'' Callum said.
Callum, who works part-time in the store while studying for a master's degree at the University of Otago, said he and his brother started to give blood about five years ago.
A couple of years after that, they were asked if they could switch to plasma.
While it was a bigger commitment, they both appreciated the opportunity to help others.
Their employer, John Moyle, said the twins knew the value of blood donation from seeing him ''barely able to walk'' sometimes.
Mr Moyle has a rare auto-immune disease which causes muscular and skin problems for which he receives a plasma-derived blood product.
Today is World Blood Donor Day, and the New Zealand Blood Service is highlighting ''secret lifesavers''.
''Blood donors don't expect praise, it's just something they do,'' communications and marketing manager Asuka Burge said.
''But the impact that such an unassuming act can have on people's lives is monumental.''
The service needs more than 168,000 units of blood a year: more than 3000 donations every week.
