It is understood the 81-year-old ex-Liberal leader visited a doctor who advised him to take an ambulance to hospital.

A spokeswoman for his office said in a statement that Mr Howard had a sudden attack of appendicitis, had been operated on and was recovering well.

"He thanks everyone for their good wishes and particularly thanks the staff at Royal North Shore Private for their excellent care."

Mr Howard was treated for a heart condition in 2015 when he had two stents inserted.

Australian Associated Press