A three-months-long drug trial has been aborted in Australia after jurors were found to be playing Sudoku puzzles rather than listening to the evidence.

More than 100 witnesses had given evidence at the trial when five jurors admitted they had been playing the game to stave off boredom.

A woman juror told Sydney District Court Judge Peter Zahra that the brain-teasers had helped keep her mind "busy" as she listened to testimony at the trial, which had cost around £500,000 by the time it was abandoned on Tuesday.

"Some of the evidence is rather drawn out and I find it difficult to maintain my attention the whole time and that (the puzzle) doesn't distract me too much from proceedings," she said.

Lawyers in the case said they had initially been impressed at seeing jurors with their heads down, apparently scribbling notes.

"We actually all thought they were quite a diligent jury," solicitor Robyn Hakelis, who represented one of the accused, told ABC radio.

"The judge had made many comments about what a good jury they were, how they were taking copious amounts of notes.

"I'm quite disappointed at the end that's not what was taking place."

The jurors were rumbled after one of the defendants - who faces a possible life sentence for conspiracy to manufacture a commercial quantity of amphetamines - noticed the forewoman completing the puzzle.

New South Wales attorney-general John Hatzistergos said that while the jurors would face no penalty, jury duty had to be taken seriously.

But it was sometimes hard to ascertain whether jurors were paying attention.

"Jurors are given notebooks in which they can write various observations in relation to the evidence," he said.

"It appears that those notebooks were used for the purposes of playing this particular game."

Drug trials were becoming longer and more technical, straining jurors' attention spans, said the vice-president of the New South Wales Council of Civil Liberties, Pauline Wright.

"They're becoming longer and longer and more and more technical with the surveillance evidence and all of those sorts of things," she said.

"So the demands on jurors are much higher and some of the evidence is, I suppose, not to put it too bluntly, quite boring.

"But jurors have to remember how hugely important the role that they're playing is."

A new jury is expected to be sworn in within weeks and the trial will resume.