The DA said on Wednesday that the constitutionality of the legislation introducing e-tolling to Gauteng will be challenged.
"This could have far-reaching consequences for its roll-out in the province," the DA said in a statement.
Details of the legal action would be announced by the party's national spokesperson Mmusi Maimane on Thursday.
Meanwhile, Outa said on Wednesday that a lack of feedback from the government was discouraging public
comments about Gauteng e-tolls.
The public had until Friday to comment on the tariffs for Gauteng e-tolls, Outa chairperson Wayne Duvenage said in a statement.
Transport Minister Dipuo Peters would then have to consider the comments, and gazette the final tariffs and the date on which tolling would start, he said.
"Outa believes there will be far fewer submissions issued from the public this time around, as previous contributions to the e-toll tariffs, on no less than two prior occasions, were graced with no feedback to the public," said Duvenage.
He said the transport department received more than 11 000 submissions in November 2012, and received verbal feedback in three public engagement sessions.
In May, proposed tariffs and draft regulations were published for public comment and no feedback was given.
In October, final regulations were published, but were not open for public comment.
Duvenage said this meant the public was unaware of the comments submitted and the minister's response.
Peters now had an opportunity to show her commitment to assessing and involving the public's opinions, by responding to the comments before publishing the final tariffs and start date.