A mystery buyer has paid nearly $1 million for a tiny portrait listed for $80,000 at a Sydney art sale, drawing gasps in the auction room as bidding soared.

Key points:

The hammer fell at $770,000 but the buyer will pay $939,000 due to the auction house's buyer's premium

The hammer fell at $770,000 but the buyer will pay $939,000 due to the auction house's buyer's premium The oil-on-cardboard painting is 22cm by 20cm and was painted in 1934

The oil-on-cardboard painting is 22cm by 20cm and was painted in 1934 The chairman of Sotheby's thought it might sell for $400,000 after pre-auction interest

William Dobell's oil-on-cardboard portrait, Woman In Restaurant, went under the hammer on Tuesday night, attracting four determined phone bidders, who pushed up the final price in $10,000 increments.

On account of its petite size of 22 centimetres by 20 centimetres, it is likely to have set a new world record for square-centimetre price for Australian artwork at auction.

"Who can predict what people's tastes are? That picture set a record when it last sold in 1962 but the world has moved on," Sotheby's Australia chairman Geoffrey Smith said.

Based on strong pre-auction interest, Mr Smith thought it might sell for about $400,000.

"It was astonishing, we were all speechless," he said.

All the bidders were private collectors, rather than institutions which sometimes pay top prices for sought-after artworks at auction. The winning bidder wants to maintain anonymity.

With the audience audibly shocked, bidding stalled at $770,000 when Mr Smith was put on hold while the buyer's agent sought more money, several long minutes later the agent came back on the line to advise they had reached their limit.

The painting Woman In Restaurant (1934) is oil on cardboard. ( Supplied: Sotheby's Australia )

The hammer fell at $770,000 but the buyer will pay $939,000, which includes the auction house's 22 per cent buyer's premium.

The price is a new record for a painting by Dobell, a three-time winner of the Archibald Prize, who died 49 years ago.

Dobell's most recognised Archibald Prize subject was painter and art patron Margaret Olley.

William Dobell's 1948 Archibald-winning portrait of Australian artist Margaret Olley. ( AGNSW )

Dobell painted Woman In Restaurant in London in 1934 and it was early recognised as one of the most collectable pictures by this very collectable artist of the early 20th century.

It was last sold in 1962 as part of the renowned collection of Isaac Norman Schurek, an ardent patron of Dobell's.

Bidding at that sale of 30 Dobell works through Lawson's auction house in Sydney was so frenzied that it has come to be recognised as the birth of the Australian art auction market.

Three bidders vied for Woman In Restaurant, which was the star of the sale, selling for 4,500 guineas, a new world record for an Australian painting at the time.

The buyer in 1962 was Joanna Dusseldorp, wife of Lendlease founder Dick Dusseldorp and the grandmother of actress Marta Dusseldorp.

The Dusseldorp family owned the picture for almost 60 years until they listed it for sale.

Woman In Restaurant was part of an auction at Sotheby's Australia's Important Australian and International art sale in Sydney where 81 artworks were listed and 62 sold for $8.6 million.