Vodafone staff have been told they can have every Friday from 2pm off, from now until the end of February.
If people work rostered hours, where they cannot leave at 2pm, they have been given a Your Day, a day in lieu to use before the end of April.
Staff were told the "summer hours" were in celebration of summer.
"We hope you'll make the most of the extra time – perhaps escape the city for the weekend, pick up the kids from school and head to the beach, or maybe catch up with friends and family around the barbecue."
Staff were told it was not just about stopping work early, and they would still be expected to be productive.
"It is about working efficiently throughout the week so you're able to wrap up any current work and leave the office after 2pm on Fridays to enjoy more of the great Kiwi summer.
"So where possible, encourage your team members to get a head-start on the weekend if they can, by taking advantage of our summer hours, and if your team is rostered, don't let them forget about the extra Your Day, as it will need to be used by the end of April this year."
It is one of a growing number of businesses encouraging workers to get the same amount of work done in fewer hours.
Perpetual Guardian led the charge with a trial that found productivity increased 20 per cent with a four-day working week.