iPhone 6 Plus. Image source: Apple.

In case it wasn't clear by now, people really like Apple's (NASDAQ:AAPL) new iPhone 6 and 6 Plus. For the past two quarters following the release of the new models -- not including the few days that the iPhone 6 and 6 Plus were available in the fiscal fourth quarter -- Apple has utterly obliterated all expectations for unit shipments.

In the fiscal first quarter, Apple moved a mind-boggling 74.5 million iPhones. The Street was only expecting 66.5 million iPhone unit sales. Apple followed that up by selling another 61.2 million handsets, again making short work of the consensus estimate of 58 million.

Source: SEC filings. Fiscal quarters shown.

Could the Mac maker have yet another blowout in store for the current quarter?

Third time's the charm
Not that Apple needs to charm investors any more than it already has, but it never hurts to follow up two excellent quarters with another one. Right now, the market is expecting Apple to ship between 45 million and 48 million iPhones in the fiscal third quarter ending in June.

One Street analyst thinks that range is too conservative. UBS analyst Steve Milunovich thinks that the company has a good chance of coming in well above current expectations, and as such, is adjusting his own estimates higher. Milunovich believes that there is sufficient demand for Apple to sell at least 51 million iPhones. His model uses data from 20 countries all over the world, as well as Google search data to gauge demand. That's not unlike Morgan Stanley analyst Katy Huberty's approach with her AlphaWise methodology that has proven to be directionally accurate in the past.

Apple's business in China continues to soar, and is now already half as big as its U.S. business. Milunovich believes that Apple's strength in China will continue. Search volumes for the iPhone in China have more than doubled as of earlier this month. In fact, Apple sold more iPhones in China than in the U.S. last quarter for the first time ever.

Not the one and only
It just so happens that KGI Securities analyst Ming-Chi Kuo has a similar estimate for the current quarter. Kuo is modeling for 51.5 million iPhones in the June quarter. Investors who follow Apple will recognize his name as one of the most accurate sources for supply chain info and unit estimates.

iPhone 6 and 6 Plus demand appears to be sustaining itself quite well. Even at the low-end of Apple's own revenue guidance for the fiscal third quarter ($46 billion), that would represent 23% growth from a year ago. That's impressive for a revenue base as large as Apple's. Besides, Apple typically comes in well above its own guidance range -- it beat the high-end of its guidance last quarter by more than $3 billion.

After the June quarter closes, Apple will enter what is typically the seasonally slowest time of year for iPhone sales ahead of the September refresh. Consumers then know that new iPhones are on the horizon, and wait accordingly. Incredibly, if Apple sells more than 51 million iPhones in the June quarter, that would be more than it sold during the holiday quarter of 2013 when the iPhone 5s was launched.