Customer service operator Sarah Veasey said she spotted a ball of fire the size of a tennis ball appear in a ceiling light fitting and raised the alarm shortly after 3.30pm.
Two Wanaka Fire and Emergency New Zealand volunteer crews and trucks attended the scene and police were called to direct traffic in Ardmore St.
Deputy fire chief Tony Wellman said the light appeared to have overheated and burst into flames.
He said the post office was full of smoke and the light was still glowing when fire crews arrived.
A constant stream of customers hoping to post last-minute Christmas cards and presents kept arriving at the post office as the drama unfolded and were stunned to find it closed.
Ms Veasey said she hoped the post office would reopen this morning.
