"Plus, it is important because it sends a sign of support for the entire Rainbow Community and any future employees."
The company said employee Storm Jury, who is an out transwoman working at Fletcher Building's Tradelink business in Australia, had welcomed the announcement.
"A business committing to this is such a huge deal for people in my community. Transitioning means different things to each person who goes through it," she said.
"It is very dynamic and individual, so having the leave to do whatever you require as part of your transition and take your own path is really helpful," Jury added.
"Not living as your authentic self, takes monumental strength because you can feel stuck, and some people even feel so trapped they feel no other option but to quit their job, even a job they love, to be able to transition."
Fletcher Building said the transitioning at work guidelines were intended to help people transitioning navigate the process, and help managers and colleagues show support and respect.
The company said it employed more than 14,700 permanent staff across New Zealand, Australia, and the South Pacific.
The 10 days gender affirmation leave is based on the person's base salary, plus any allowances, excluding overtime.
The leave was intended to give employees paid time off to attend medical, and other, appointments related to their transition.
Employment New Zealand said gender transitioning was a unique and personal experience.
"Employees should discuss with their employer what steps they are intending to take within the workplace and what this means for other staff and possibly clients," the agency advised.
"The employee can bring a union delegate or other person along for support."
The leave will be in addition to any sick leave or other leave entitlements and is effective from today.
Employment NZ said it was good practice to develop these steps into a written action plan.
It said such a plan could help employers and employees agree how to manage any issues around an employee's transition and how the employer might support the employee.
The University of Otago already had gender transitioning at work guidelines.
The guidelines covered topics for people who were transitioning, and for those who had a transitioning team member.
It also discussed how employees could meet legal and university obligations, and pointed out that the Human Rights Act prohibited discrimination on the grounds of sex.