The Australian Retailers Association said yesterday that sales were still down across the board despite major retailers offering heavy discounts usually only seen after December 25, reported The Daily Telegraph.

"A number of retailers went on the record saying they didn't want to discount this year in the lead-up to Christmas but they have been forced to," executive director Russell Zimmerman said.

Interest rate increases and the rising cost of living are mostly to blame but Mr Zimmerman said the cool, wet start to summer was also taking its toll on seasonal items such as clothing.

Retailers were still holding out hope yesterday that shoppers would spend up big on last-minute gifts after receiving their holiday pay.

"Some of the major centres have been requesting their merchants to stay open right through what is basically a 36-hour period of trade," Mr Zimmerman said.

"It's nothing unusual, it has happened for the past four or five years, but it is growing in popularity."

Parramatta's Westfield shopping centre is in the middle of a marathon trading session, with stores staying open overnight and until 5.30pm today.

The Australian Retailers Association predicted $6.97 billion would be spent on post-Christmas sales from Boxing Day to early January, up 3 per cent on last year.

"Retailers would probably like me to say there's going to be a bigger increase in that but we just don't believe that to be the case," Mr Zimmerman said.

Among the retailers cashing in on the 36-hour trading marathon yesterday was the Sydney Fish Market at Pyrmont.

Marketing manager Louise Shaw said traders at the market had so far been spared the downturn experienced elsewhere.

"People might be skimping on presents and things, but when it comes to food on the day of celebration I think most people want it to be special and don't mind to splurge a bit," she said.

Shoppers at the fish markets will buy a staggering 83kg of prawns, 93kg of snapper and almost 1000 oysters every minute during today's traditional rush.

Ms Shaw said prawns ranged in price from $15/kg for imported varieties to $38/kg for local king prawns.

Whole snapper was selling yesterday for $11-$19/kg, oysters were $12-$19 a dozen and smoked salmon was going for about $30/kg.