Two African National Congress (ANC) MPs who failed to toe the party line during Tuesday's vote on the Protection of State Information Bill may yet face disciplinary action.

At face value they violated party discipline, but it was still necessary to examine the circumstances that led to this "unfortunate situation", ANC Chief Whip Mathole Motshekga told journalists at Parliament on Thursday.

On Tuesday, veteran ANC MP Ben Turok left the Assembly shortly before the vote, while Gloria Borman abstained during the electronic vote.

The ANC had imposed a three-line whip – compulsory attendance – on its MPs for the vote.

Motshekga said it was concerning that the ANC had learned of the two's objections to the bill through the media.

"It is worth noting that both comrades have never raised any objection to the draft legislation in at least four caucus meetings the ANC convened to afford all MPs an opportunity to discuss and receive clarity on the draft bill," he said.

"They have also never raised their concern with any structure or with any leader of the organisation.

"It is regrettable that they should defy [an] agreed organisational decision and then raise their concerns with the media."

Party discipline required all ANC members not to define themselves outside the democratically agreed positions.

Motshekga said he briefly met Turok and Borman on Thursday afternoon, but had yet to receive a full explanation from them.

Both had stated they had not wanted to address the media about the matter. Turok said he had been "ambushed" by the media and had no alternative but to respond.

It also appeared that both MPs were relying on an old draft of the bill, from 2010, which did not contain the many amendments made that were included in the final draft.

This might have led them into "temptation" to defy the party line, he said.

The ANC's parliamentary leadership would immediately institute an investigation to establish the facts of the matter.

Should the information gathered in the investigation warrant disciplinary action, the matter would be referred to the ANC disciplinary committee.

Regarding the other 32 ANC MPs who were absent from the vote, Motshekga said he was in possession of a number of apologies.

These ranged from ill health to international travel and unavoidable executive commitments.

Each case would be examined individually to determine its authenticity followed by appropriate action where necessary, he said.