Qantas chief executive Vanessa Hudson promised hefty investment in new planes driving up its capital expenditure bill next year will lift long-term returns for shareholders, after delivering a 13 per cent reduction in first-half profit.

Investors sold off Qantas shares heavily, sending the stock down 6.4 per cent to $5.23 after analysts flagged downgrades to full-year earnings, given its profit is typically skewed to the first half, and as the airline flagged more spending to improve customer service. Qantas will miss margin targets for its international business.