Australian bank St George has accepted a 19bn Australian dollar (£9.2bn; $18bn) offer from Westpac, creating the country's largest finance firm.
Westpac said it was confident neither the government nor competition regulators would derail the takeover.
Analysts suggest the deal will shake up Australia's banking sector, with more consolidation expected.
The firms say the move will allow the enlarged firm to make savings and get better access to funding.
However, critics of the deal say that customers will have less choice of where to bank and will lose out in the long run with higher fees and poorer service.
Unions also expressed concerns about potential job losses.
Regional buy-up
Westpac is Australia's third-biggest bank, while St George is the fifth largest.
The merged bank would become the country's largest by market capitalisation, worth about A$66bn.
It would also become the country's largest mortgage lender - with a market share of about 25%.
Analysts suggest that it may lead to other large financial institutions, including the Commonwealth Bank and National Australia Bank, looking to expand by purchasing regional banks.
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