"Mrs Bush extended her regrets in a telephone conversation yesterday to Mrs Janette Howard, wife of Prime Minister John Howard," her spokeswoman, Sally McDonough, said.
"Mrs Bush has been treated with physical therapy for a pinched nerve, which she injured while hiking earlier this year. Her physicians have strongly advised her to not travel overseas in order to avoid potential complication and aggravation by extended long distance travel," said Ms McDonough.
Mr Howard said earlier this month that Mr Bush will arrive in Sydney on September 4, two days earlier than scheduled, and return to Washington while the September 7-9 Asia-Pacific Economic (APEC) summit is still underway.
Mr Howard said Mr Bush's schedule bent to accommodate the release of a critical September 15 progress report on the war in Iraq. White House officials have declined to address speculation that Mr Bush will go to Iraq before the report.
Mr Bush also wants to be back in the United States ahead of ceremonies marking the sixth anniversary of the September 11 terror attacks on New York and Washington, Mr Howard said.