Fines are being reduced for some speeding motorists in South Australia but demerits points will double, under changes announced by the SA Government.

The current fine is $260 for being caught exceeding the speed limit by less than 10 kilometres per hour.

That will be reduced to $150 but two licence demerit points will be recorded.

Fines will also be reduced but demerit points increased for speeding offences 15-45 kph above the limit.

SA Police Minister Jennifer Rankine thinks the higher demerit points will be an effective deterrent to speeding, as drivers could face the prospect of losing their licence twice as fast as before.

SA Treasurer Jack Snelling said the changes were calculated to be revenue neutral.

"As Treasurer I would be more than happy to see a reduction in revenue from speeding infringements, but for that to happen there needs to be a reduction in the number of speeding drivers," he said.

"Increasing the fines for drivers blatantly disregarding the law and increasing demerit point across the board will hopefully do just that."

Professor Mary Lydon, of the Centre for Automotive Safety Research at Adelaide University, says increasing demerit points recognises the higher risk of injury from higher speeds.

"For ... average drivers who would exceed the limit between nought and 10, the licence is a very valuable item and so the demerit points [change] it's not a reduction in penalty, it's really a serious penalty and serious deterrent to those people," she said.

The RAA is supportive of the changes, says official Wendy Bevan.

"What we want to see is a system of fines and penalties which accurately reflects the risk. This system they've brought in which has demerit points but a reduction in fines for the lower speed offences seems to be a commonsense approach," she said.

Police data shows 38 per cent of speeding fines issued last year were for offending by less than 10kph above the limit and about 79 per cent were for speeding by less than 15 kph above the limit.

New fines from September 2012 Speeding but less than 10kph over limit - $150 fine and 2 demerit points

Speeding but less than 10kph over limit - $150 fine and 2 demerit points Caught 10-20 kph over limit - $330 fine and 3 demerit points

Caught 10-20 kph over limit - $330 fine and 3 demerit points Caught 20-30 kph over limit - $670 fine and 5 demerit points

Caught 20-30 kph over limit - $670 fine and 5 demerit points Caught 30-45 kph over limit - $800 fine and 7 demerit points

Caught 30-45 kph over limit - $800 fine and 7 demerit points Caught 45 kph or more over - $900 fine and 9 demerit points Each offence also attracts a $60 Victims of Crime levy