Investors are beginning to show signs that any further intervention by Japan to support the embattled currency will have less of an impact than before.

One-week implied volatility in the dollar-yen has fallen to trade well below highs seen last month, even as the currency pair closes on ¥145.90 per dollar — a level that triggered a near $20 billion intervention from the Finance Ministry. The Japanese currency has fallen for eight weeks in a row, its longest-losing streak since May.

"I’m actually surprised dollar-yen vols are as low as 10.1% given all the focus on central banks prepping for currency management/intervention,” wrote Pepperstone Group head of research Chris Weston in a note Sunday. "With dollar-yen making new highs the prospect of the Ministry of Finance intervening to halt the yen move is elevated.”

Yen traders are also waiting for key inflation data from the United States due Thursday. Last month’s consumer price index print sent the yen lower as it reinforced bets of continued aggressive rate hikes from the Federal Reserve.

Japan’s currency has tumbled more than 20% against the dollar this year due to its widening monetary policy differential with the U.S. It hit ¥145.90 per dollar on Sept. 22, sparking intervention from Japan.

The Finance Ministry likely financed last month’s intervention by selling short-dated U.S. securities like T-Bills, strategists at Bank of America including Shusuke Yamada wrote in a note Friday. Eventually the prospect of having to use deposits or slightly longer-dated Treasurys would raise the hurdle for further intervention, they said.

"Dollar-selling intervention is limited by the amount of foreign reserves held and considerations to liquidity of the reserve assets and to the impact on the U.S. rates market Japan’s intervention would have,” they wrote. "We do not believe the ¥145-146 level is a line in the sand and, if the move is not too rapid, dollar-yen may be allowed to break this level.”