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."

Blackmores soared more than 500 percent last year as its profit jumped on demand from Chinese buyers willing to pay a premium for goods made Down Under. Other Australian niche exporters have also tumbled since the tax changes were unveiled, with Bellamy's Australia Ltd, which makes organic baby formula, falling 8 percent on Tuesday.

The Chinese government said in March it would remove a so-called parcel tax that was previously levied on imports sold online. Rates ranged from 10 percent on food and infant items that came into the country through free trade zones to 50 percent on cosmetics and alcohol.

Instead, it will charge value-added and consumption duties that are currently imposed on most products sold in China but with a 30 percent discount, according to a Ministry of Finance statement.

"Historically, there have been two different import methods: Personal use articles subject to the postal tax and the trade of goods subject to duties and consumption tax," Dolly Zhang, director of indirect tax and business advisory services for Deloitte China in Shanghai, said last week. Now there's effectively a third category.