AMP, which handed out free shares to 170,000 UK policyholders when it de-mutualised last June, warned that profits for the the full year were vulnerable to falling markets.
George Trumbull, group chief executive, said forecast profits of at least A$774m (pounds 275m) for the full year were on track. But he added: "Volatility in the markets, however, makes forecasting very difficult."
Profits before exceptionals in the first half were at the top end of analysts' expectations at A$603m, helped by strong British investment markets which rose 13 per cent. But the company said it was "cautious" about the second half.
Shares on the Sydney stock market closed down 34 cents at A$21.88, slightly more than the general weakness in the Australian market. Analysts expressed disappointment at smaller-than-expected cuts in costs.
AMP owns Pearl Assurance, one of the UK's largest life insurers. But only policyholders with London Life and AMP UK have benefited from free shares.
The insurer - Australia's biggest - has been -particularly acquisitive in the UK. In April, it completed a takeover of Henderson, a UK fund manager, boosting its funds under management by A$57bn to A$172bn (pounds 62bn).
At the beginning of last year, the insurer came second to the Prudential in the race to buy Scottish Amicable, the UK life insurer which specialises in pensions sold through independent financial advisers.
It has open ambitions for further acquisitions in its home markets of Australia, New Zealand and the UK. A spokesman said: "It is fair to say we are looking out all the time and that the UK is a very important market to us. We missed out on ScotAm but got Henderson this year. We have made it clear we do have an acquisitive stance."
AMP also has a 50 per cent stake in Virgin Direct, Richard Branson's life insurance operation launched over two years ago. Funds under management at Virgin Direct rose from A$2.78bn (pounds 990m) to A$4.31bn (pounds 1,530m) in the first half.
AMP confirmed a substantial proportion of UK shareholders had already sold their shares, most of whom are believed to have held shares in trust. But more than half have kept their shares since the flotation last June.