One of the world's biggest fund managers has emerged as a significant shareholder in a2 Milk with a 5.03 per cent stake.

In a notice to the NZX, New York-headquartered BlackRock said recent purchases had taken it from 4.99 per cent to over the 5 per cent threshold, thereby requiring it to declare its stake.

A2 Milk earlier this year become New Zealand's largest listed company by market capitalisation after announcing another bumper profit and the formation of a joint venture with the world's biggest dairy exporter, Fonterra.

The company's net profit swelled to $98.5 million for the six months to December 31, up 150 per cent on the previous comparable period.

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Most milk contains the A1 and A2 beta protein but a2 Milk specialises in milk from cows that produce just the A2 protein. A2 Milk says its products help people who have trouble digesting standard milk.