The South Australian Government has conceded more must be done to lift NAPLAN results, after the latest scores revealed South Australia is the worst performing mainland state across most categories.

Education Minister Susan Close has announced $67.5 million will be spent over four years to boost results in the state's struggling public primary schools.

Raw results from the May test, released today, show South Australia is the worst performing mainland state in 16 out of 20 categories.

Average numeracy scores improved, but average writing scores were down by almost 2.5 per cent.

Ms Close said there was "no doubt" the state needed to improve its performance.

"There are some good signs in the data ... but it's not enough," she said.

"Improvements do cost money [and] we have a $70 million injection over the next four years for our public primary schools, for more teachers and better strategies.

"We'll take NAPLAN data and other school data and identify public primary school students who need assistance, and they'll get it."

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Principals defend education standards

The Primary Principals Association said South Australia was not a poor performer, and NAPLAN testing was given too much attention.

Association president Pam Kent says the statistics were a small part of what the education system delivered.

"NAPLAN is important... it covers the basics, but there is a heck of a lot more to student learning than NAPLAN," she said.

"That tends to be a frustration between teachers and principals because there's an inordinate focus on NAPLAN."

But Opposition education spokesman John Gardner said the results made clear South Australia was well below the national average.

"It's hardly conceivable but on a number of measures South Australia's results have gone backwards — even on last year's poor results," he said.

Nationally, reading and numeracy scores have improved since 2008 but writing scores have declined since 2011.

Queensland and Western Australia have shown the most improvement since 2008.