Piper Jaffray analyst Gene Munster predicts that Apple’s upcoming earnings call could show lower Mac sales than originally expected by investors, due to the expected imminent arrival of a new MacBook lineup.

Munster came to this conclusion after the latest US data from NPD suggested that Mac sales were less than expected during the March quarter, up just five per cent, Apple Insider reports. Investors have said that they expect Apple to announce a 14 per cent increase year over year, with sales of 4.3 million units during the most recent quarter, but NPD’s latest sales data suggests they’ve over estimated. (More below)

The reason for this, Munster believes, is that Apple hasn’t yet launched its rumoured MacBook Pro and MacBook Air overhaul, meaning potential customers are holding off for a lineup refresh. Munster says that MacBooks account for 50 per cent of all Apple’s Mac sales.

Munster now predicts that Apple will announce sales of around 4.1 million to 4.4 million Macs, which is a nine to 17 per cent increase from the same quarter of 2011. He says that Apple will focus on strong iPhone and iPad sales at its earnings call on 24 June, as Mac sales are expected to account for just 15 per cent of Apple’s overall revenue for the March quarter.

If the MacBook Pro, MacBook Air and iMac are updated within the coming months, Munster thinks that Apple’s Mac sales will recover from any lower than expected sales figures reported at the earnings call.

It is widely believed that Apple is set to refresh its MacBook line-up before the end of April, with a thinner, MacBook Air-like pro expected to make an arrival alongside Intel’s latest Ivy Bridge processors.