The company said its revenue guidance for the year took into account the planned transition to new infant formula packaging during the fourth quarter.
"Gross margin percentage for the full year is expected to remain broadly consistent with the first half, given the benefit of throughput efficiencies and currency movements," a2 Milk said in a statement to the NZX.
The group's total marketing investment is now expected to be in the range of $82m to $87m for the full year, given higher expenditure primarily in the US and China businesses in second half compared to the first half.
Harbour Asset Management senior research analyst Oyvinn Rimer said several other high-growth stocks - particularly ones like a2 Milk that trade on high price earnings multiples - had seen their share prices drop over the last six months after issuing guidances that failed to match optimistic market expectations.
"When they don't meet market expectations, they get a caning by the market," he said.
Even after today's fall, a2 Milk has had a spectacular run; the stock traded this time last year at just $3.56.
A2 Milk, which this year became New Zealand's biggest company by market capitalisation, last year reported net income of $90.6 million for the 12 months to June, up from $30.4m a year earlier.
Revenue in that year rose 56 per cent to $549.5m.