The Novopay debacle is being blamed for delaying the release of National Standards data - an excuse described as odd by a leading unionist.
Education Minister Hekia Parata said last month the national schools performance reports would be available by the end of June.
Parata's office and the Education Ministry now say results could be weeks away.
They claim the Novopay problems have caused some schools to be late in forwarding financial information, which has led to the hold up.
"We initially thought we might be in a position to release the data at the end of June, but now want to take some more time to make sure we've got it right," Parata's press spokeswoman said on Friday.
"The last time, the information wasn't released until late September so we are still tracking well ahead of that."
The Education Ministry claims issues with Novopay have made it difficult for some schools to finalise financial accounts needed for the National Standards reports.
"The effort schools have put into submitting their annual reports, which includes National Standards and Nga Whanaketanga RumakiMaori results for publication, is appreciated," the ministry's deputy secretary, Rowena Phair, said.
"Our regional staff are working with these schools to identify how we can best help them.
"We are processing schools' annual reports as they are received and will release last year's National Standards and Nga Whanaketanga Rumaki Maori information by regional council, territorial local authority and individual school level in the coming weeks."
NZEI immediate past president Ian Leckie cast doubts on the Government's claims about Novopay causing delays.
"Most schools had submitted all the relevant information by March, so there has been plenty of time to get the National Standards information published," Leckie said.
"Using Novopay as an excuse for the data being late is, to say the least, odd.
"School finances are not annual accounts, and are no reflection on national achievement."
He suggested an online assessment tool known as e-asTTle would be a more plausible reason.
"A more likely explanation for the information being late is that the ministry also recalibrated the e-asTTle results, which means all kids have been marked down in their writing, which has probably skewed all the figures."