Four Sidney Nolan paintings from the Art Gallery of New South Wales (AGNSW) have fetched more than $700,000 at an auction in Sydney.

This far exceeded auction house Sotheby's previous expectations of $340,000 to $470,000.

Nolan's Burke Lay Dying was the most contested artwork, selling for $336,000.

Sturt on the Riverbank fetched $174,000.

Earlier, gallery director Edmund Capon said his team had been reassessing Nolan's collection since 2000.

He said the funds raised from the auction will be used by the AGNSW to buy other Nolan paintings.

Sotheby's head of paintings, Georgina Pemberton, says the works are part of Nolan's explorer theme.

"Three of the four works are from the collection of Lord Alistair McAlpine, which were gifted to the gallery in the 1990s," she said.

"The money raised will be used to buy new works and in Lord McAlpine's memory."

Ms Pemberton says the works were displayed in Melbourne for a week before moving to Paddington Town Hall in Sydney.

"There has been a lot of interest in these paintings," she said.