Caltex Australia has warned its first-half profit will more than halve from last year's, sending its shares tanking as the company grapples with a slowing economy and pressure on its refining margins.

The petrol station giant suffered its sharpest share-price drop in 30 years on Thursday, with the stock plunging 24 per cent to a five-year low in early trading after Caltex warned its half-yearly profit could come in as low as $120 million, citing margin pressure, the economic slowdown, and softer demand from sectors such as transport and construction. That's down from $296 million the year before.

"The downgrade is enormous," said Jun Bei Liu, a portfolio manager at Tribeca Investment Partners.

In a statement to the ASX, Caltex chief executive Julian Segal explained that "the industry continues to experience difficult macro-economic conditions arising from the slowing Australian economy, low refining margins and high crude prices".