Participation in South Australia's council elections is already up by 16,000 postal votes compared to the same period last time, with just days until ballot boxes close.

Deputy Electoral Commissioner David Gully urged people to post their ballot papers today or by tomorrow at the latest to ensure they were received by 5:00pm on Friday and counted.

"As of 4:00pm last night, we had 265,386 [postal votes] back, and that's tracking at around 22 per cent of the total at the moment," he told ABC Radio Adelaide's Breakfast program.

"At the last election we had about 249,000 at the same period during 2014, so 16,000 less than what we have at the moment."

Mr Gully said country and regional councils had the highest proportion of participation so far.

Kimba has already had 56 per cent of its ballots sent back, while Southern Mallee has received 48 per cent.

"But Mitcham council in the metropolitan area is up around 30 per cent at the moment with a few days to go, so that's pretty encouraging."

The Electoral Commission of SA (ECSA) said 664 positions were up for election this year across 68 councils.

The Local Government Association of SA said ECSA received 1,375 nominations for the election, or 41 more than in 2014, and it was the highest number of nominations since 1995 when the state had 115 councils.

Ballot paper secrecy 'upheld'

Mr Gully said while the reply-paid envelope included flap with a barcode and space to sign a declaration with your date of birth, returning officers would remove the flap once the envelopes were received.

The anonymous envelopes would then be forwarded to a machine for opening before being counted by humans.

"The secrecy of the ballot is upheld and no-one's ballot papers will be looked at while the certificate's still in the envelope," Mr Gully said.

People could also place their ballot papers in a box at their local council office.

Mr Gully said all mayoral votes would likely be counted by Saturday, while the wards would take a little longer and were expected to be counted by Wednesday.