Western Cape Premier Helen Zille's apology for tweets she made about colonialism was merely a ploy to keep her position as premier, the African National Congress (ANC) in the Western Cape says.

Zille and Democratic Alliance (DA) leader Mmusi Maimane held a joint press conference on Tuesday to announce she would be stepping down from all her party positions following tweets she posted on March 16.

She would remain as premier of the province until her term ends in 2019.

Her "unreserved apology" though, was "disingenuous and an insult" to South Africans' intelligence, ANC provincial secretary general Faiez Jacobs said.

"The apology that Zille has given is clearly just a manoeuvre to try to get the DA out of the hole it has dug for itself," Jacobs said on Tuesday.

"The apology is nothing more than a cynical face-saving strategy due to the polls indicating that they are losing black voters because of the party's stance on race, racism and colonialism."

'Callous, arrogant, insensitive'

Zille refused for three months to acknowledge that she deeply offended many black South Africans, he said.

"The truth is that Zille realised that she would be kicked out as premier of the Western Cape if she went the route of the party's disciplinary process and so decided to strike a deal with Maimane in which she remains in her position as premier.

"Does the DA really expect South Africans to believe that it does not have confidence in Helen Zille to the extent that she has been banned from party activities, but has confidence in her to lead a provincial government on its behalf?"

Maimane has compromised his leadership, and the announcement was really about making him look as if he is in charge, he continued.

"If he is in charge, he should fire her as premier and expel her from the party for her callous, arrogant, insensitive and cruel utterances.

"The reality is that he cannot precisely because at the end of the day, he must do his masters' bidding."

He claimed Zille's pro-colonialism remarks "go to the heart of the DA's philosophy, policies and attitude to our people's history and their current marginalised existence".

"Preserving privilege and the ill-gotten gains of centuries of oppression and exploitation is the DA's priority," Jacobs added.

'Zille must go'

ANC Western Cape shadow premier Khaya Magaxa said: "It is clear the DA does not have the political will to decisively and properly deal with Zille or the outcry by affected people.

"The Zille agreement to keep her as premier of the province, does not absolve her from the continued hurt she caused not only to the citizens of the Western Cape, but also the legislature and her high office.

"She has caused massive loss of public trust."

The provincial ANC called on Zille to be removed as premier, as her position and attitude to key issues was "untenable and unsustainable".

They would protest outside the Western Cape legislature on Thursday calling for her removal.