With shrinking prospects at home and the threat of further yen weakness, Japanese companies are rushing to buy overseas and seem willing to pay top dollar, as shown by Japan Post's $5 billion bid for Australia's Toll Holdings.

Over the long term, Japan's demographics give a bleak prognosis for domestic demand. The population has been falling for a decade and is projected to drop from 127 million to 87 million by 2060, 40 percent of whom will be over 65.

But bankers and analysts say a more immediate impetus to the dash for overseas growth is the fear, in an era of deflationary pressure and huge monetary stimulus from Japan's central bank, that the weak yen will fall still further, making overseas targets more expensive if buyers do not strike now.