SYDNEY, Australia — In May, she made headlines around the world by becoming the first Australian to breast-feed a child in the federal Parliament, for which she was praised by Sheryl Sandberg, the chief operating officer of Facebook.

But on Tuesday, the Australian politician, Larissa Waters, a senator and deputy co-leader of the country’s Greens Party, announced that she was “devastated” to be resigning after she disclosed her previously unknown Canadian citizenship.

Under Section 44 of the Australian Constitution, citizens of other countries are not eligible to run for federal Parliament.

“It is with great shock and sadness that I have discovered that I hold dual citizenship of Australia and Canada,” Ms. Waters said.