Two Service SA centres earmarked for closure by the State Government are among the busiest in the state, according to data released under Freedom of Information laws.

Key points: The closures of the Prospect, Modbury and Mitcham Service SA centres were flagged in the budget

The closures of the Prospect, Modbury and Mitcham Service SA centres were flagged in the budget The Government says moving more services online is about 40 times cheaper

The Government says moving more services online is about 40 times cheaper The Opposition says it will make it harder for people to pay money to the Government

Treasurer Rob Lucas announced plans to close centres at Modbury, Prospect and Mitcham in his September budget, citing the need for savings across all government departments.

But data released to the Opposition shows the Prospect and Modbury Service SA centres have recorded some of the highest customer visit numbers in the state over the past two years.

The customer service centres are routinely used for tasks such as applying for driver's licences and proof of age cards, renewing vehicle registration and paying bills.

Shadow Treasurer Stephen Mullighan said the data was evidence the Government had not "thought through" the decision to close the centres.

"What we have here is a saving … which hurts South Australians who are just trying to find an easy way to pay money to the Government," he said.

"These are essential service centres for people who don't have access to the internet, for the elderly, for people who might need some help understanding forms … and for people who might not have English as a first language.

"It is farcical that they are closing these Service SA centres, without realising the impacts on the local communities who rely on them."

The Modbury Service SA customer centre is one of three earmarked for closure. ( ABC News: Isabel Dayman )

The move is set to save the State Government $18.8 million across the forward estimates, with the possibility of further closures already foreshadowed.

A date is yet to be set for the closure of the Prospect, Modbury and Mitcham centres.

Customer usage of Service SA centres Customers served in centres: 2017-18 2016-17 Elizabeth 141,730 140,979 Marion 117,617 113,065 Prospect* 105,103 104,330 Modbury 104,519 103,880 Seaford Meadows 98,480 95,645 Port Adelaide 98,219 99,426 Tranmere 85,022 76,016 Mitcham 83,182 78,205 Gawler 80,751 80,457 Adelaide 74,004 88,792 Murray Bridge 71,982 76,026 Mount Gambier 67,590 67,972 Regency Park 59,436 55,639 Berri 53,551 53,933 Kadina 47,848 48,381 Port Pirie 45,687 45,595 Port Augusta 39,099 39,932 Port Lincoln 37,107 39,544 Whyalla 36,651 36,789 Naracoorte 27,268 27,373 *Bolded centres earmarked for closure

Transport and Infrastructure Minister Stephan Knoll said the budget had been full of "tough decisions", and the three centres had been selected based on "geographical" considerations.

"For the people at Prospect, it's being able to go to Regency Park or the CBD … for the people at Modbury, it's the Elizabeth or Tranmere centres … and for people at Mitcham, [it's] either getting to Tranmere, Marion or the CBD," he said.

"So, it's more about where they [customers] are in proximity to other centres, rather than the sheer numbers at a centre."

SA Treasurer Rob Lucas revealed the cuts in his first budget in 17 years. ( ABC Radio Adelaide: Malcolm Sutton )

Mr Knoll said customers who were unable to use online services, or did not "feel comfortable" doing so, would still be able to access services through other Service SA centres, post offices, or over the telephone.

"We made some tough decisions in the budget and we stand by those decisions," Mr Knoll said.

"At the moment, the cost of someone going into a Service SA centre is about $17 and to provide that service online costs about 40 cents.

"That's 40 times the cost to taxpayers to provide a Service SA centre … so [this] means we can spend money on other worthwhile projects."

Last week, it was revealed Member for Adelaide and Cabinet minister Rachel Sanderson — whose electorate includes the Prospect centre — wrote to Mr Knoll after the budget, to pass on the concerns of her constituents about the closure.

Mr Mullighan said the State Government should immediately reverse the decision.

"They've got this wrong … [and they] obviously had no idea how busy and how popular those centres are, and how many people are going to be affected by closing them," he said.

"The Government had six months to develop this budget [and] it just shows that they didn't use that six months to think things through."