The DA, which won 22 percent of the vote in the last general election, hopes to make major gains in the 2019 vote, but it has struggled to shed its image as a “white” party.
South Africa remains deeply divided more than 20 years after the end of white-minority apartheid rule, as stark racial inequality leaves millions of black people with poor housing, education and job opportunities.
Mmusi Maimane, the DA’s first black leader, said Zille’s social media posts “in connection with colonialism undermine our reconciliation project”.
“Our public representatives must, at all times, be sensitive to the legitimate anger that people still feel about our past and its legacy,” he added.
Zille, who is the current premier of Western Cape province, vowed that she would not “plead guilty to charges of misconduct which I never committed”.
On Saturday, the DA initially announced that she was suspended before changing its position to say a final decision would be made after she has been given the chance to defend herself.
In the contentious tweet, which was sent in March, Zille wrote: “For those claiming legacy of colonialism was ONLY negative, think of our independent judiciary, transport infrastructure, piped water.”
She has 1.14-million Twitter followers.
The DA, which promotes a liberal, equal-opportunity message, is pushing hard to broaden its appeal among black voters.
But it has been bruised by social media scandals as the party tries to take advantage of the declining popularity of President Jacob Zuma and the ruling African National Congress (ANC).
Maimane, 36, was fast-tracked through the party ranks by Zille, 66, who was his predecessor as leader.
The DA has its roots in the now defunct Progressive Party, co-founded by the late anti-apartheid activist Helen Suzman in 1959.
South Africa was colonised by both the Dutch and British. The Dutch East India Company established the first colony in 1652.