The head of Blackmores Ltd, the Australian vitamin maker whose shares are heading for the steepest two-day drop since 1987, said investors are overreacting to China's tax changes on imported goods bought online.

Chinese authorities last week announced a list of products that will be subject to new e-commerce tax rules, amid an overhaul of its system aimed at making levies on products posted to shoppers from overseas more comparable with rates paid locally. Blackmores fell as much as 19 percent in Sydney on Tuesday.

"People assume the worst," Chief Executive Officer Christine Holgate said on Tuesday in Beijing. "There is nothing on those lists today that I can see that gives us any concern."