The workplace regulator is investigating a top law firm for a second time over workplace fatigue and the hours worked by staff, but the firm's managing partner, Danny Gilbert, has told staff long hours are "a reality of life" for corporate lawyers.

The firm, Gilbert + Tobin, has already been subject to an earlier SafeWork investigation last year, regarding the hours worked by staff and allegations staff were forced to take supplements to keep up with the extended working hours and sometimes slept at the office.