Zip Co said on Friday it had "identified and contained" the fraud event, which involved co-ordinated individuals establishing new accounts using legitimate credentials to execute unpaid transactions.
The attack resulted in $US7.2 million ($10.1 million) in cash operating expenses, the Sydney-based company said.
The event also disrupted transaction volumes and customer growth due to mitigation measures taken to contain the incident.
Zip has notified the authorities and lodged a claim with its insurer.
Zip Co's president for its US operations, Joe Heck, resigned earlier this month, in a move the company announced on October 16.
He was replaced on an interim basis by US chief technology and operations Rory Herriman, effective immediately.
Zip Co shares closed Friday at $1.46, down 29.5 per cent from Thursday and down 63.6 per cent year-to-date.
Zip said it had returned to growth in the US in September and had 4.7 million customers there as of September 30, up 1.7 per cent from the end of June.
But its revenue from the US grew 11.1 per cent to $259.7 million, with Australian revenue up 1.7 per cent to $111.5 million.
Net bad debts in the US grew to 1.93 per cent of total transaction volume, from 1.51 per cent.
In Australia, net bad debts dropped to 3.51 per cent, from 3.83 per cent.
RBC Capital Markets analyst Julian Mulcahy said the results were a beat on most metrics, but the loss rates were higher and the fraud event would impact profit.
Zip Group chief executive and managing director Cynthia Scott said the results showed customers were choosing to use Zip more often, and the company was well-positioned for the holiday season.
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