Aldi Australia's sales are forecast to rise almost 90 per cent to $15 billion over the next four years - challenging the Woolworths/Coles duopoly - as the discounter opens stores in new states and consumers become more willing to spend their grocery dollars at the discount retailer.
Aldi's sales rose 60 per cent to $8 billion between 2013 and 2016, according to broker Morgan Stanley. Sales are expected to reach $15 billion by 2020 as Aldi lifts store numbers from 373 to 628 and as its market penetration (the percentage of grocery shoppers who visit each month) rises from 36.8 per cent nationally to 54.9 per cent. 
In comparison, Coles' food and liquor sales are forecast to rise 15 per cent to $37 billion and Woolworths' to rise 10 per cent to $46 billion by 2020.
Aldi's share of the national food and liquor market is expected to rise from around 6 per cent to 10 per cent, eclipsing that of Metcash's IGA network and placing more pressure on Woolworths and Coles to cut prices to defend their turf.
"Clearly the headwind for the Australian supermarkets has only just begun," said Morgan Stanley analyst Tom Kierath.
Morgan Stanley's forecasts exceed even the most optimistic forecasts of UBS, which estimated in 2014 that Aldi's sales could reach at least $9.3 billion by 2019 and as much as $13 billion if the retailer fixed problems such as checkout queues, out of stocks and the quality of its fresh food.
The new forecasts reflect not only Aldi's rapid store expansion but its growing acceptance among grocery shoppers as consumers seek to reduce the cost of their weekly shop.
According to Roy Morgan Single Source figures released last week, 5.3 million Australians visit Aldi stores in a four-week period, compared with 10.5 million at Woolworths, 10 million at Coles and just over 4 million at IGA.
Morgan Stanley says Aldi customers are now spending an estimated $100 over a four-week period, up 66 per cent from an average spend of $60 in 2007.
The average shop at Aldi is less than half that at Woolworths, Coles or IGA stores because the discounter has a narrower and shallower product range, offering about 1350 stock keeping units compared with around 25,000 at the major chains.
However, the value of the average shop at Aldi has grown faster than that at Woolworths (up 18 per cent) and Coles (up 22 per cent) over the last eight years, while the average shop at IGA stores has fallen 10 per cent.
"Aldi customers tend to start out buying staples, products that have a very low risk of failure, such as sugar, toilet paper, rice and dishwashing liquid," Mr Kierath said.
"After these products have met consumers' expectations, they buy more products, moving into other dry grocery lines," he said. "Again, after these expectations have been met, the last category customers graduate to is fresh food."
Aldi has expanded its product range over the last few years, adding more national brands to its private label range and, like Coles and Woolworths, has improved its fresh food offer to enable more customers to complete more of their grocery shop rather than just stock up on staples.
Morgan Stanley believes Aldi's penetration across the eastern states is 47.7 per cent and its national market penetration is 36.8 per cent. The broker expects Aldi's East Coast penetration to reach 60.3 per cent and its national penetration 54.9 per cent in 2020, by which time it is expected to have opened at least 95 stores in South Australia and Western Australia and another 160 stores in the eastern states.
Aldi was unable to comment on the forecasts before deadline, but managing director Tom Daunt has previously played down speculation that Aldi's sales could double or &shy;triple, saying that despite the retailer's rapid growth since it set up shop in 2001 it remained "somewhat of a niche retailer" with a limited range sold at heavily discounted prices.
Aldi named the locations of the first of the 70 stores it plans to open across Western Australia. The first four stores will open in Mirrabooka Square, Kwinana, Belmont and Lakeside Joondalup on   June 8.
Other confirmed locations include the upmarket suburb of Nedlands, as well as Armadale, Mandurah, Mandaring, Ellenbrook and Rockingham. Up to 20 stores are expected to be open by the end of the year.