WELLINGTON, New Zealand--Infant formula marketing company a2 Milk Ltd. said the pandemic lockdown in Australia's Victoria state has disrupted surrogate-shopping sales of its products to mainland Chinese consumers.

The New Zealand company on Monday said a contraction in so-called "daigou" sales was larger than it had expected. That would result in "materially below plan" sales for New Zealand and Australia in the first half, it said.

The company expects a significant improvement in overall performance in the second half, helped by growth in China, and continues to see an operating profit margin of about 31%.

It forecast full-year revenue of 1.8 billion to 1.9 billion New Zealand dollars ($1.18 billion to $1.24 billion). First-half revenue was forecast at NZ$725 million to NZ$775 million.

Write to Stephen Wright at stephen.wright@wsj.com