The a2 Milk Company chief executive David Bortolussi has a tough job ahead as he tries to stabilise the baby formula maker after its fourth earnings downgrade in nine months.

Mr Bortolussi is hunting for a replacement for a key executive, head of Asia, Peter Nathan, who is leaving after 14 years with the Kiwi company. A2 Milk also flagged a review of its key China business and a blowout of more than $NZ100 million ($92.9 million) in provisions for old stock.