SYDNEY—Reserve Bank of Australia Governor Philip Lowe on Friday said the central bank was becoming increasingly uneasy about the potential for inflation to jump in the U.S., fanned by the country’s “problematic” fiscal expansion.

“I am less relaxed. It is highly unusual to have such stimulatory fiscal policy when the economy is already operating at a very high level of capacity,” Mr. Lowe said in testimony before parliament.

“One...