* Weaker China sales to hurt profit in second half
* Blackmores says it is reviewing its operations in China
* Shares mark biggest daily percentage drop since 1985 listing (Recasts with outlook, adds shares, comments)
SYDNEY, Feb 19 (Reuters) - Australian vitamin maker Blackmores Ltd cautioned that weak sales in China, its biggest export market, will drag on its net profit in second half of the year, driving down its shares the most since the company listed three decades ago.
The vitamin maker, which has benefited from exploding Chinese demand for Australian health products, said sales to the mainland fell more than a tenth in the six months to December, and the pattern was continuing partly due to "a general softening of consumer sentiment."
Full-year revenue is expected to be "modest," it warned, even as the beachside Sydney-based company turned in a record first-half net profit. Blackmores said it was reviewing its operations in China, without elaborating.
The warning sent Blackmores shares down as much as a third, their biggest percentage drop since 1985. The stock hit its lowest intraday level since 2015 as investors rethought the underlying value of the company's main growth prospect.
By midsession the shares were down 23 percent, while the broader market was up 0.4 percent.
Blackmores' dour outlook underlines the precarious position of companies around the world that have staked their future on insatiable Chinese consumer appetite.
In January, computer maker Apple Inc rattled global markets as it issued a surprise revenue warning based citing weaker demand for its iPhones in China.
That is all against a backdrop of bitter trade tensions between the United States and China which have led to tougher conditions for exporters.
"You've got a property market over there that's very weak, you've got all these trade tensions, you've got the lowest economic growth in 20 years in China (but) for Blackmores that space was supposed to be growing no matter what," said Steve Johnson, chief investment officer at Forager Funds Management.
"They're almost certainly buying more of that type of product, it's just that when the demand increases so does the competitive response."
Blackmores reported a net profit of A$34.3 million ($24.46 million) for the six months to Dec. 31, up 0.4 percent from the previous corresponding period. Half-year revenue from ordinary activities rose about 11 percent to A$319.4 million, which the company said was its best-ever revenue figure for the period. ($1 = 1.4025 Australian dollars) (Reporting by Byron Kaye in SYDNEY and Aby Jose Koilparambil in BENGALRU; Editing by Himani Sarkar)