New Zealand's largest telecommunications firm Spark posted a 12.7 percent rise in interim net profit on Thursday, despite a modest growth in mobile and broadband revenues.

Spark said net profit for the six months to December 31 was NZ$178 million (US$128 million), up from NZ$158 million a year previously.

Chairman Mark Verbiest said group revenue was up 4.1 percent at NZ$1.8 billion, a "pleasing" result in a challenging and competitive market.

"It is clear the intense ongoing price competition, particularly at the lower end of the market, is driving margin pressure," he said.

Revenue from mobiles lifted 4.4 percent and broadband was almost static at 1.5 percent. But IT services to corporate and government clients outperformed to rise 19.3 percent.

Spark shares were down 4.31 percent at NZ$3.55 in early afternoon trade in an overall market that was off 1.01 percent.