It means a day's work. In the UK, darg has mainly been Scots and northern English usage, though it did appear in Life in the London Streets by Richard Rowe, published in 1881: “He must go out bone-grubbing; but even his dull face showed, or seemed to my fancy to show, that, his dreary ‘daily darg’ got through, he wanted to hide in a hole.” However, Mr Rowe spent some years working for the Scotsman in Edinburgh, so probably picked it up there.
It was taken to Australia and New Zealand by emigrants. Though Rowe spent 14 years in Australia from 1853 on, it’s unlikely he heard it there, as it began to appear in print in both countries only in the 1920s. It has now fallen out of favour once again — the Australian Dictionary Centre included darg in a list of words in 2000 for which it would like printed evidence, noting that it had none after 1978. Some has since come in, but it is clearly rare these days.
The darg referred not to how much work you could do in a day, but how much was considered a reasonable day’s work, or one’s allotted or fixed share of work for the day. A New Zealand journalist, Fred Miller, who wrote a column in the old Southland Daily News with the title The Daily Darg, noted in his book Ink on My Fingers in 1967: “Theoretically, when you have finished your darg you go home.”
The darg was often a matter of dispute between workers and bosses, as Dan Stalker explained by e-mail: “The challenge for Australian managers in the 60s and 70s was ‘lifting the darg’. New technology meant more could be done in less time, while staff were resistant to ‘upping the darg’.”
The OED says that it’s a half-swallowed version of daywork, a day’s work, especially the amount of land that could be ploughed in one day (hence a close equivalent of acre).