DBS Bank had picked up on the accounts and fund flows linked to 1Malaysia Development Berhad (1MDB), but it had believed there were no illicit activities involved.
DBS was one of the banks named by the Monetary Authority of Singapore over "instances of control failings" and "weaknesses in the processes for accepting clients and monitoring transactions" related to 1MDB.
Mr Gupta said the bank had conducted its due diligence.
"A lot questions were asked, a lot of due diligence done, but our teams on the business were satisfied," he said. "In hindsight, were the accounts part of a complex (network)? Probably. Could we have done better in figuring it out? Of course we could have."
These lapses happened mainly three years ago, and since then, standards at DBS "have gone to a different level".
"We already tightened up a lot compared to 2013, partly because the MAS has also been tightening the standards over the last two to three years," said Mr Gupta.
"Last time, we would look at transactions and find them reasonable. Now, we ask two more questions. But there is still work to do."
The 1MDB investigations are ongoing, and it is unclear if DBS will be hit by a fine, Mr Gupta added.