Bill Shorten says Prime Minister Malcolm Turnbull took too long to remove Stuart Robert after he was found to have breached ministerial standards on a 2014 trip to China.
Mr Robert quit the frontbench on Friday, and the opposition leader says the human services minister left only because of "political reality" and the government had tried to avoid sacking him.
"I didn't think that our Liberal ministers needed it explained to them that their job is to work for the people of Australia, not to squire rich, fat-cat Liberal donors around the capitals of the world," Mr Shorten said in Sydney.
Mr Robert has been under fire all week and it was announced he would not be considered in the impending ministerial reshuffle.
The investigation found Mr Robert had an interest in a company that signed a mining deal at a ceremony in Beijing, which he attended.
Martin Parkinson, the prime minister's departmental secretary who investigated the issue, concluded Mr Robert had acted "inconsistently with the statement of ministerial standards", although the minister "may not have intended to do so".
The trip was taken with the approval of former prime minister Tony Abbott.
The resignation will give Prime Minister Malcolm Turnbull another position to fill as he considers a ministerial reshuffle during the weekend.
Conservative ministerial colleagues have defended Mr Robert, exposing a rift within the government.
Nationals leader Barnaby Joyce said he could not see any wrongdoing.
"What we have here is allegations he met people," Mr Joyce said.
"If meeting people in China is a crime then every politician in this building is gone."
Treasurer Scott Morrison earlier in the week said it was a "ridiculous beat-up" by the Labor opposition and media.
Media reports suggest Mr Morrison, a close friend of the minister, tried to save Mr Robert's job when the issue was considered at a meeting on Thursday night.
Labor frontbencher Chris Bowen said there was "open warfare" between Mr Turnbull and some of his front bench.
"What we've seen is Scott Morrison going into bat for internal factional reasons, trying to save the career of a minister who had clearly, in an open-and-shut manner, breached the ministerial code of conduct," Mr Bowen told reporters in Sydney.
Labor argued the minister breached standards banning him from acting as "a consultant or adviser to any company, business, or other interests, whether paid or unpaid, or provide assistance to any such body, except as may be appropriate in their official capacity as minister".