"We do consider that to be internal information that is confidential... and has been our policy for a very long time," DA federal congress convenor and presiding officer Greg Krumbock told reporters in Johannesburg.
"We do have a very strong emphasis in our party on incentivising vote gathering activities rather than membership."
Krumbock said this was because the DA wanted its members to focus on interacting with the people of the country rather than building membership.
"It's not to say we discourage membership, but we want people to be out there in the communities."
Krumbock was asked about the party's membership ahead of its national elective conference which will be held next month.
Despite not giving exact figures, Krumbock said the DA's membership had tripled since 2009.
"We have a very impressive membership database now and it's growing very, very fast. But we are more excited that our votes are growing a lot faster," he said.
About 1 650 delegates would attend the DA's national congress.
Krumbrock said there would be a minimum of 760 public representatives, which comprised MPs, MPLs, and councillors, and a further 851 ordinary DA members at conference.
This meant there were 505 more delegates than at its last congress.
"This is a reflection that our party has grown since the last congress. One in four people now vote for us and that is reflected in our membership," he said.