Australian MP Bill Shorten said John Key had been "a good friend to Australia".
"I wish him and his family all the best," he tweeted.
Australian ambassador to the United States, Joe Hockey, said Key had done the country an "outstanding service".
"Your country is stronger and richer," he tweeted. "I really enjoyed working with you."
The Australian's political editor Dennis Shanahan said Key had broken another political rule by resigning while at the top of his game.
"Typically, John Key is breaking another political rule; he's resigning as Prime Minister without being pushed," the writer said.
He went on to detail the "remarkable ten-year-run" Key had first as leader of the National Party then as Prime Minister.
Shanahan said the Kiwi Prime Minister was resigning at a point in his political career when many Western leaders were just starting to approach the top.
He's said Key's political style was that of a "successful businessman" who would work with minority parties in coalition.
Most media outlets made reference to the "surprise announcement" of the long-time leader who has hinted family reasons and his wife's asking him to call it a day were key factors behind his decision.
As the news broke, the majority have chosen to reflect on his popularity in office as well as the toll it's taken on his family life.
The New York Times said it was a move that has "stunned the nation".
The Huffington Post described it as an "unexpected move".
Le Monde made reference to how the National Party would have to deal with his unexpected announcement as it looked for a replacement in the coming days.
