The South African National Roads Agency Limited (Sanral) has rejected claims by the Opposition to Urban Tolling Alliance (Outa) that it was fabricating e-tag sales figures.

"The e-tag sales figures we have released are accurate," Sanral said in a statement on Thursday.

"As a state-owned entity, everything we do is subject to audit processes by both the Auditor General and our external auditors. We have no reason to 'fabricate' these figures as Outa alleges."

Outa chairman Wayne Duvenage said on Wednesday research showed that only 15% of freeway users were tagged.

Earlier on Wednesday, Sanral spokesman Vusi Mona said: "Currently 890 388 VLN/e-tags have been committed."

Outa's research showed, of a sample of 2098 cars which used the freeway, 317 had e-tags, which equalled around 15.1%.

Of a sample of 2 236 cars which did not use the freeways, 212 had e-tags, or around 9.5%.

Duvenage said seeing that about a third of cars in Gauteng did not use the freeway, it was expected the non-freeway figure would be lower.

Applying the sample to the total number of cars which used Gauteng's freeways every month, around 2.3-million, Outa believed the number of e-tags sold was only around 350 000.

"E-tags are easy to see, especially at traffic lights on freeway off and on-ramps and in car parks of shopping centres," Duvenage said.

"This Christmas, there is a new game for your kids while travelling around Gauteng -- spot the e-tag."

Outa called on Sanral to come clean and provide the actual e-tag count passing under the toll gantries.

Sanral said it would not attach too much value to Outa's research unless the sources of the information were known and found to be sound.

"We don't know what equipment has been used, with which technical specifications and standards it complies and where it has been installed," it said.

The e-tolling of Gauteng highways came into effect last Tuesday.