The union, however, claims there will be significant delays to customers.
First Union organiser Tali Williams said the impact could be worse in places with fewer banks - such as small towns - where there would be fewer staff to carry the workload.
"The impact is in [the] number of staff who are available to carry this work out.
"So say for example in a city where there's lots of different branches, the impact may be [not as bad as], say, in a town where there's one branch and those guys are going out."
Ms Williams said Christmas Eve was traditionally a busy day for banking and customers.
"As they rush around to get their last-minute shopping done, perhaps go into a bank branch to get money out - so those people may be impacted, and same with those who want to do a bit of online banking."