Some methods are perfectly legal, like investing in real estate elsewhere, buying businesses overseas and paying off debts owed in dollars. Others, like Smurfing, are more dubious, and in certain cases, outright illegal. Chinese customs officials caught a woman last year trying to leave the mainland with $US250,000 strapped to her chest and thighs and hidden inside her shoes.

A woman seen through a window prays at Longhua Temple in Shanghai, China, as fears of financial instability rattles the nation. Credit:Bloomberg

While individuals are limited to moving $US50,000 a year across China's borders, companies and sophisticated investors have more freedom to send out money legally for big-ticket purchases and investments. Overseas and domestic companies, which maintain bank accounts in various currencies, can also shift their cash, as well as borrow based on which currency they think will fall in value.

Yet unofficial methods abound. Companies have inflated trade invoices to keep more profits outside the country, although Chinese authorities have cracked down on the practice. Two years ago, the government gave permission for insurers to invest 15 percent of their assets overseas, up from 1.5 percent. But China abruptly told insurers this winter to suspend many of their overseas plans, according to Hong Kong financiers.

Beijing has restricted the withdrawal of renminbi from overseas branches of Chinese banks. In Shenzhen, banks have begun requiring that residents make reservations up to a week in advance if they want to change the daily maximum of $US10,000 worth of Chinese currency into dollars.