Bank staff had worked till late in the night and on holidays in the weeks after Prime Minister Narendra Modi announced the ban on Rs 1,000 and Rs 500 currency notes on November 8 last year. That decision sucked out 86 per cent of currency notes in one go, leading to serpentine queues of people spilling out of banks.
Inside, officials had to work long beyond their eight-hour shifts to cater to the rush of people.
"But later, they were treated very shabbily... exploited though Prime Minister Modi had gone on record to appreciate the good work put in by bank staff," said CH Venkatachalam, general secretary, All India Banks' Employees Association (AIBEA).
Mr Venkatachalam said there has been a long-standing pact between bank employees and their employers that staff should be paid additional money for extra work put in.
"But there is hardly any bank that has delivered... most banks just paid a fraction of the overtime allowance due to the employees," he told NDTV.
The union leader added that the employees hadn't made up their mind on the action. A strike was one possibility, he said. Approaching the courts was another.
"We are exploring our legal options," Mr Venkatachalam - who had been a sharp critic of demonetisation that he had previously described as a poorly-implemented decision - added. Unions of bank employees say they had taken up the delay in full payment of the overtime dues with the centre's chief labour commissioner and the Indian Banks Association too.
PM Modi was widely hailed for his assault on tax evasion but long queues outside banks, a cash crunch and fears of the impact on businesses and economy had led to concerted attack from its critics.
The attacks became shriller after the central bank announced in August that 99 per cent of the banned currency notes had returned to the banking sector. The government had initially estimated about 5 lakh crores wouldn't be declared following the sudden move.
This week, former union minister Arun Shourie blamed the economic slowdown on PM Modi's shock outlawing of high-denomination notes a year ago. "It was the largest money-laundering scheme ever, conceived and implemented entirely by the government," he said.