The group, that describes itself as a global community that fights against the growing power of corporations, said construction of the mine, which is to be built by the Indian company Adani, could only be stopped if the funds for doing so were unavailable.
"The only thing standing in the way of this is that Adani, the company behind the mine, still needs cold hard cash. And you can help make sure Westpac won’t give Adani a single dollar," SumOfUs said in a media release.
The group said Westpac spent millions on advertising itself as a caring, sustainable bank, "so we know it will hate this ad attacking its clean image".
Both the Queensland and federal governments approved a 31.5km rail line and temporary camp for the $21.7 billion Carmichael mine project.
Green groups had tried to fight the plans in court but failed.