Rioting has destroyed many buildings in the Tongan capital
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About 150 soldiers and police officers from Australia and New Zealand have arrived in Tonga following an appeal for help to restore order after riots.
At least eight people died when crowds ransacked much of the centre of the Pacific nation's capital, Nuku'alofa.
The city is now said to be calm but a state of emergency has been declared. The unrest was sparked by concern about the slow pace of democratic reform.
The government has now announced major changes ahead of elections in 2008.
Australia has sent about 85 of the troops and police officers deployed, with New Zealand providing the remainder.
Australia's Prime Minister John Howard and his New Zealand counterpart Helen Clark announced the mission at a joint press conference in Vietnam, where they are attending a regional summit.
"Being the largest and wealthiest countries in the region, part of our responsibility is to help," said Mr Howard.
Soothe tensions
Many buildings in Nuku'alofa have been destroyed in the rioting, including the government and prime minister's offices, the power company, Nuku'alofa's only bank and a number of Chinese-owned shops.
The trouble has been blamed on gangs with links to pro-democracy groups, says the BBC's Phil Mercer in Sydney.
They have been agitating for political reform in a country where the king wields enormous power.
The embattled government of Prime Minister Fred Sevele has now announced sweeping reforms, which it is hoped will soothe tensions in Nuku'alofa, our correspondent says.
The intervention by Australia follows similar action in the Solomon Islands and comes amid fears of a military coup in Fiji, our correspondent adds.
Australia is concerned that trouble in the region could be exploited by extremists or drug traffickers.
Reform plans
The rioting began after the government assembly failed to pass democratic reforms before it went into recess.
The Tongan king appoints 10 of the 14 cabinet members to their posts for life, with the other four reserved for "nobility".
Hereditary noblemen hold the majority of seats in the country's parliament.
After the death of King Taufa'ahau Tupou IV in September, a government committee recommended that all lawmakers be elected by the public.
King Tupou IV reigned for 41 years and was opposed to reforms.
His son and heir, Siaosi Tupou V, is thought to be more in favour of change.