Want to buy Apple Inc.'s (NASDAQ:AAPL) new supersized iPhone 6 Plus? Be prepared to wait a while. Hours after iPhone 6 presales launched on Friday, 5.5-inch iPhone 6 Plus units quickly sold out, pushing orders back three to four weeks for all models.

But, surprisingly, the 4.7-inch iPhone 6 has continued to be readily available for the Sept. 19 launch date in the U.S. as of 9 a.m. EDT.

The story is similar in the U.K. and Australia, where the iPhone 6 Plus is also sold out. The smaller iPhone 6 is still available, but shipping estimates have increased to seven to 10 days after the launch date of Sept. 19.

The sell-out of the iPhone 6 Plus indicates strong demand, at least stronger than Apple expected when it allocated inventory. What we don't know is how many units Apple allocated or what that means in total presales.

If the iPhone 6 Plus remains scarce, it could trigger another version of last year's 5s “gold rush,” where Apple severely understocked gold iPhone 5s models in its retail stores on launch day after underestimating consumer demand.

While actual sales data isn't yet available, an AT&T spokesman told Re/code that demand for the iPhone 6 overall is tracking stronger than for the iPhone 5.

Analysts expect both iPhone 6 models to sell between 75 million and 80 million devices in its first three months, which would surpass every iPhone launch in history along with a number of other Apple products.  

If you were one of the people who weren’t able to secure an iPhone 6 or 6 Plus model that will ship on launch day, your best bet now is to either wait in line at an Apple Store or your respective carrier’s store. But another alternative could be visiting a retail store such as Target (NYSE:TGT) to make reservations for the iPhone 6 and 6 Plus in person.

The iPhone 6 officially goes on sale at 8 a.m. on Sept. 19 in Apple retail stores, carrier stores and select retail locations.