Cambodia's government has been getting away with murder. Not the kind of genocidal slaughter conducted by the Khmer Rouge in the 1970s.

Nor the scale of killing that has been roiling Syria, or that has put Ukraine, Venezuela, Thailand and Bangladesh in the global headlines of late. But murder nonetheless, with Cambodian citizens deliberately targeted by their country’s security forces.

On January 3, five striking garment workers were shot dead in Phnom Penh while peacefully demanding a minimum livable wage. Many others were severely injured by gunfire and beatings.

More than 20 have been detained without trial. This followed deadly violence against unarmed demonstrators protesting against last year’s deeply flawed national election, won, yet again, by Prime Minister Hun Sen’s ruling Cambodian People’s Party, which has dominated political life for more than three decades.

The recent killings repeat a pattern of political violence that has recurred all too often at crucial moments in Cambodia’s history – even after the 1991 Paris Peace Agreements, which were intended to bring not only peace, but also democracy and human-rights protection to the country’s long-suffering people.

No country in the world deserved all three more, ravaged as it was for two decades by massive United States bombing, civil war, a genocidal reign of terror by the Khmer Rouge, invasion by Vietnam and more civil war – with some two million dead as a result.

Hopes were high that Cambodia had been set on a transformative path by the success of the United Nations peace plan, the huge peacekeeping operation that followed and the remarkably peaceful election of 1993 (in all of which Australia played a leading role during my time as Foreign Minister).

In some respects, those hopes have been realised. The Khmer Rouge did wither away, and with it the lingering threat of renewed civil war. The Cambodian economy – strongly supported in recent years by aid and investment from China – has grown steadily (though it lags behind most of its regional neighbours, and concerns about corruption and political instability are preventing it from reaching its full potential).

But Cambodia’s record on democracy and human rights since the Paris Peace Agreements has not been a good one. A grenade attack on an opposition rally led by Sam Rainsy in March 1997 killed 16 people and injured more than 100.

That July, after an uneasy period of sharing power with Prince Norodom Ranariddh’s royalist party, Hun Sen launched a bloody coup in which his opponents were exiled, arrested, tortured, and in some cases summarily executed.

Neither episode generated much international reaction: Hun Sen still had enough political capital from his fight against the Khmer Rouge and his cooperative role in the peace process, while Sam Rainsy was regarded as deeply flawed, and the royalist leadership as feckless.

Cambodia-fatigue among policymakers also played a role. At the time, I wanted to believe that these reverses would be temporary, and there were too many like me.

Since then, while preserving a democratic façade, Hun Sen has ruled, for all practical purposes, as an autocrat, showing scant regard for rights of free expression and association – and resorting to violent repression whenever he has deemed it necessary to preserve his and his party’s position.

This has been accompanied by staggering levels of corruption, with Cambodia ranked 160th by Transparency International, out of 175 countries.

There are stories, unverifiable but plausible, that 20 or more of Hun Sen’s closest associates have each amassed more than $1 billion through misappropriation of state assets, illegal economic activity and favouritism in state procurement and contracting.

There has also been political patronage bordering on parody, with one recent count putting the government’s size at 244 ministers and secretaries of state.

For far too long, Hun Sen and his colleagues have been getting away with violence, human-rights abuses, corruption, and media and electoral manipulation without serious internal or external challenge.

But things are beginning to change. A credible new opposition party – the Cambodia National Rescue Party – has emerged under Sam Rainsy (who looks a little more like a national leader now than just a monochromatic anti-Vietnamese crusader), gaining significant popular support.

A large number of social-media-savvy young voters have been turning out in the streets demanding a change of government.

Internationally, too, some pressure has been building, but not enough.

Ed Royce, Chairman of the US House Foreign Affairs Committee, has called on Hun Sen to resign; condemnatory resolutions have been passed in a number of parliaments; and many states entered criticisms of one kind or another into the record when the UN Human Rights Council reviewed Cambodia’s human-rights record in Geneva a few weeks ago.

But the tone of too many of these statements has been muted. Australia’s statements have been typical – falling over backward to avoid giving offence, and too anxious to balance criticism with praise.

Australia’s new foreign minister, Julie Bishop, has talked, as foreign ministers often do, of the need to avoid unproductive “megaphone diplomacy” and to “engage, not enrage” her counterparts.

But, it seems that no robust critique was delivered when she met privately with Hun Sen in Phnom Penh on February 22 – even though Australia’s high standing in Cambodia (not least owing to its historical role in the peace process) means that its voice certainly would have been listened to.

There is a place for quiet diplomacy that relies on genuine engagement to encourage significant behavioural change. But when states behave badly enough for long enough, loud megaphones can also be in order.

I know Hun Sen and worked well with him in the past. I have resisted strong public criticism until now, because I thought there was hope for both him and his government.

But their behaviour has now moved beyond the civilised pale. It is time for Cambodia’s political leaders to be named, shamed, investigated and sanctioned by the international community.

PROJECT SYNDICATE