Sabine Leutheusser-Schnarrenberger, a member of the Liberal party (FDP), told the Rheinische Post that buying the data was currently a grey area legally but amounted to theft.
"Buying this data is a murky and highly problematic area both ethically and morally speaking but also judicially," she said.
"Someone who copies data onto a blank CD unbeknown to his employer is indisputably guilty of theft."
The German region of North Rhine-Westphalia declared earlier this month that it would continue to buy the data, considering it a useful way of helping trace fraud.
"Purchasing such data is legal. That has been decided by the highest legal authority. Therefore we will continue to buy such data," Hannelore Kraft told the Bild newspaper.
The data purchased recently indicated that Swiss banks were offering clients tips on how to transfer cash away from Switzerland to Asia before a planned tax deal between the two countries comes into force, said Kraft, according to Bild.
The two neighbours have been embroiled in a spat over tax since 2010 when German authorities raided branches of Credit Suisse bank in 13 German cities after buying data on suspected tax frauds.
Switzerland reacted angrily, saying the data were stolen in violation of its banking secrecy laws. As much as €180 billion ($222 billion) in German assets are hidden in Switzerland, according to unconfirmed media reports.
A tax deal between the two countries, aimed at ending such disputes, is to take effect in January 2013 but still needs to be ratified by both parliaments.
The double taxation agreement, signed by ministers earlier this year, would see German citizens with assets parked in Switzerland's notoriously secretive banks paying a tax rate of 26.4 per cent on these holdings.
But opposition lawmakers in the German upper house, the Bundesrat, have threatened to block the deal.