First Union finance secretary Andrew Casidy said the loss of weekends was an example of how "we are making huge changes in the way society works without even thinking about it".
The union's general secretary, Robert Reid, said New Zealand work patterns had "dramatically changed" since the 1980s.
"There were laws passed to allow Saturday trading in retail and we were told Sundays would never be allowed. Then the laws changed to allow Sunday trading."
Reid said up until 1991, when the Employment Contracts Act came in, those working in retail on weekends were paid penal rates, but the act "destroyed all the retail awards" and retail had become a 24/7 world.