The Bank of Japan should expand already-record stimulus at its April meeting, as a wait-and-see stance risks further appreciation of the yen, according to Takatoshi Ito, a former colleague of BOJ Gov. Haruhiko Kuroda.

"It would be better for the BOJ to act in April," Ito, currently a professor at Columbia University in New York, said in an interview. "If they opt to monitor economic conditions, the yen may gain more and the BOJ is likely to be forced to add easing in June in response to the markets' demands. Then, the effects of stimulus would be limited."

The central bank is in unfamiliar territory, with it's new negative rate policy generating criticism and unable to lift stocks and weaken the yen like the two previous rounds of stimulus did. Kuroda defended the policy this week, saying markets would've been worse off without it, while board member Yutaka Harada said it was too soon to judge its effects.