New domestic sales data from the NPD Group ) says Apple’s U.S. Mac sales were up 18% year-over-year during the holidays, suggesting that the company remains on track to report its best quarterly Mac sales ever.

In a note to clients — as reported by “AppleInsider” ( Piper Jaffray analyst Gene Munster unveiled the latest stats from the NDP research group. The figures showed that international Mac sales are growing even faster than domestic, which means Apple is in line to meet Wall Street expectations of 24% year-over-year sales growth.

NPD data from the September 2010 quarter hints that Mac sales were up 22% year-over-year, while Apple reported actual Mac unit growth of 27%. Munster thinks Apple will sell 1.2 million iMacs in the December quarter, without a major redesign, compared to about 500,000 of the newMacBook Air.

As it did in December, the latest NPD data shows that Apple remains on pace to sell as many as 4.3 million Macs in the December quarter. If that proves accurate, it would be Apple’s best quarter ever for Mac sales, notes “AppleInsider.” The previous record came in its last financial quarter, when the company sold a record 3.89 million Macs.