It's time for the CEO to walk the plank.

Faced with a boardroom mutiny, Australian chief executives are finding life at the top tougher - and shorter. The country is seeing unprecedented CEO churn, with six chieftains of top 100 companies biting the dust in the past six months.

As economic growth slows and share markets sour, corporate directors are under shareholder pressure to keep a sharp eye on management and take decisive action to forestall a downturn in the company's fortunes, headhunters say.

The directors are taking their lessons from a slew of high-profile corporate collapses in the United States and at home. The implosion of energy trader Enron Corp and others showed many boards were asleep at the wheel as their companies dived.

"Whenever you get a big turn in market fortunes, it exposes poor management and business models," says Deutsche Asset Management fund manager Lawrence Grech. "What may have been acceptable as mediocre returns get exposed as poor returns."

The latest surprise exit came from Australia's biggest company, BHP Billiton Ltd/Plc. CEO Brian Gilbertson quit just six months into the job because, a source told Reuters this week, he proposed a major deal that would have unseated the company's current chairman.

Gilbertson's departure followed the resignations of five other chief executives at top 100 companies - fund manager and life insurer AMP Ltd, construction group and property fund manager Lend Lease Corp, health care group Mayne Ltd, banking and insurance group Suncorp Metway Ltd, and surf-wear maker Billabong Ltd.

Among smaller companies, upmarket retailer David Jones Ltd and once-promising biotech research group Biota Holdings Ltd also ditched their chief executives.

The eight CEOs were in their jobs for between six months and seven years. While statistics aren't kept, headhunters say a CEO's tenure appears to be shrinking to three to five years.

Excluding BHP Billiton and Suncorp Metway, the premature departures followed long periods in which the groups' shares sagged compared with their peers and the broader market. The Australian market fell 12 percent last year.

"The market and boards of companies will tolerate one to two years of underperformance and beyond that management's tenure is at risk," says Alliance Capital Management's head of research John Bergin.

The failure of Enron and WorldCom in the United States and Australian insurer HIH also has given corporate governance a front seat, forcing non-executive directors to be more proactive.

"Quite often you get boards - less so now - that are not as well informed as they might be, so that when things do not come out to expectations, there might even be almost a sense of panic that they need to be seen to be doing something," says Korn/Ferry International senior client partner Peter Van de Velde.

At AMP, sliding share markets eroded the reserves of the group's Pearl life insurance business in Britain and prompted the exit of CEO Batchelor after three years at the helm.

At David Jones, the whiff of losses from an expansion into food retailing led directors to push out CEO Wilkinson.

A botched centralisation strategy at Mayne's private hospitals cut the legs from once-vaunted CEO Smedley and a 43 per cent slide in Lend Lease shares over two years cost CEO Higgins his job.

Fund managers said CEOs who faced challeging times this year include media chiefs - such as Fred Hilmer at Australia's second largest newspaper publisher, John Fairfax Holdings Ltd.

Media sector shares fell twice as far as the overall market last year, thanks to a weak advertising market, and CEOs must show they can boost returns as the market rebounds.

Cyclical industries might also face tougher times as Australia's strong growth starts to slip.

"Some chief executives have been carried by the economy, and you'll only find out how good they are when things turn tough," says 452 Capital director Peter Morgan.

Companies facing testing times include building materials groups Boral Ltd and CSR Ltd, and appliance retailer Harvey Norman Holdings Ltd, which all thrived in an Australian home building boom that has now peaked.

Losing a chief executive means finding a new one. In most cases last year - Lend Lease and Suncorp Metway were exceptions - directors chose their new CEO from within company ranks. Fund managers, however, see no clear trend towards improved succession planning.

In some cases it had more to do with directors wanting a quick appointment to minimise uncertainty in the market, rather than embarking on an extensive search for the best person.

"An executive search can take six months plus, particularly for Australian companies looking for executives with some international experience," said Alliance Capital's Bergin.

Internal replacements also are far cheaper, but if boards are looking for an overhaul, external replacements are more likely to sweep out old management, says Owen Thomas, director, executive remuneration, with corporate advisers RPC Group.

Fund managers might clamour behind the scenes for heads to roll, but they are unhappy about the fat cheques paid out in severance packages for high profile CEOs.

Gilbertson's package, still under negotiation, was reported to be up to A$30 million (US$18 million), including his pension.

Morgan, known as a vocal fund manager now heading his own niche fund, says companies should not be allowed to hide the details of fat payouts.

"It's sad that the rewards at times seem to be fairly out of kilter with what's the norm, especially in a closed market like Australia, where a number of industries are dominated by monopolies and duopolies," he says.