The diary of a Gallipoli digger that describes the historic campaign in vivid detail has sold for $24,000 in a Sydney auction.

The well-preserved, pocket-sized diary is now in the hands of an overseas bidder who wishes to remain anonymous.

New Zealand Private George Petersen landed with Nelson Company on April 25, 1915, the day that has since come to symbolise the sacrifices made by Australian and New Zealand servicemen and women as Anzac Day.

His diary includes fascinating accounts of the Gallipoli landings themselves and the ensuing operations, with entries dated from January 1 to December 1915.

The entry for May 30 says: "On fatigue from 4:00am till 8:00am at Quinns: Turks blew up our trenches just as we arrived, terrible explosion blew dust and stones hundreds of feet in the air, landed all around us, 4 killed and 10 wounded.

"German aeroplane about again. Went from swim in afternoon. Went around to Machine Gun to see Spotty, went for a swim with him arrived back at my bivouac about 8:30 pm. Received a letter from Isabel."

Lawsons Auction House chief executive Martin Farrah says there was a lot of interest in the diary from private collectors and institutions.

"These diaries are incredibly hard to find, whether they're World War I, II or Vietnam or Boer, and this thing is written by a New Zealander which is even rarer," he said.

"It's only the size of a Bible, like the little Bibles you used to get at school. It's been through it all, through the trenches, he wrote in it every day and it's still in immaculate condition.

"It's all written in ink with those early fountain pens where you would have to dip it and you can see sometimes halfway down the page where the ink's fading and he's had to dip his pen in the ink again."