Bushfires raging on the outskirts of the south-eastern Australian city of Sydney are the worst it has experienced for up to 30 years, a senior official has said.

No immediate let-up is anticipated and firefighters from neighbouring states are being drafted in to relieve exhausted colleagues in New South Wales.

There are hundreds of houses out there that had fire to their back doorstep Senior fire official

About 3,000 firefighters, many of them volunteers, have been tackling about 70 bushfires to the north, west and south of the city.

Multiple fires suddenly erupted on Wednesday, and within hours stretched for hundreds of kilometres around the city of four million.

Giant water-bombing helicopters were brought in to help, as homeowners battled flames with garden hoses and wet towels.

Although fires are natural to the arid bush in Australia - which is suffering one of its worst droughts in 100 years - some of the blazes are thought to have been started deliberately.

Police have charged an 18-year-old student with lighting a fire on Wednesday, and there have been appeals for people to report anyone seen starting a blaze.

"The immediate environments of Sydney have probably not faced a threat like this for 20 or 30 years," Rural Fire Service commissioner Phil Koperberg said.

"We've either had fires in the north or the south or the west, but rarely have we had fires effectively along the length and breadth of the corridor which is Penrith to Sydney city."

Firefighters take water wherever they can

Mr Koperberg added that properties will remain under threat for some time given weather conditions.

More temperatures of 30C and strong winds are predicted for Friday, as many fires continued to blaze out of control into Thursday night.

A weather department spokesman said there would be trouble in New South Wales until it rained or "when there's nothing left to burn".

Although one caller to a radio programme spoke of some rain in the Blue Mountains area to the west of Sydney, other residents to the north said they were being forced evacuate their homes as walls of flames approached, Reuters news agency reported.

"There's a wall of fire coming across the ridge. We've got to go now," one male caller said.

The state authorities said about 800 people were spending Thursday night in emergency shelters because their homes were threatened. Some schools have also been closed.

The fires have thrown up huge plumes of grey and brown smoke over parts of the city and left the streets smelling of wood smoke.

'Shaken'

About 20 houses have been destroyed but only one death has been reported so far - of a 73-year-old man said to have suffered a heart attack while leading horses to safety.

Some roads have been closed, and train services and power supplies have been disrupted.

Residents have been evacuated from the areas most in danger and some schools have been shut.

The BBC's Dominic Hughes in Sydney says the speed and ferocity of the bush fires now burning in northern and southern suburbs has shaken even the most experienced of the 3,000 firefighters trying to save homes.

Preventive measures are being taken

The worst-affected area so far appears to be the north-western suburb of Glenorie, where about 15 houses have burned down.

The view from one house - of which only the chimney was left standing - appeared "like a blackened battlefield of burnt trees and sooty boulders", Reuters reported.

In Glenorie, a terrified kangaroo was reported to have sought refuge in a swimming pool, while flames leapt 60 metres (200 ft) and oil-filled eucalyptus trees exploded in the intense heat.

December and January tend to be the worst months for bush fires, and firefighters fear this is the beginning of one of the most dangerous seasons they have ever faced.

Last Christmas, a wall of flames ringed Sydney, burning through 777,000 ha (1.9 million acres) and destroying nearly 200 homes.

You can see it in the sky and you can smell it in the air Rob Hardy, Sydney, Australia

You do not have to live in one of the bushfire areas of Sydney to be affected by this situation. You can see it in the sky and you can smell it in the air. The radio is always on in the office, as those who live in the outskirts of the city listen to find out if the fires are near where they live. You hear a commotion down the corridor, as someone has to head off home because their area is to be evacuated, and they want to save what possessions that they can. The main ones being pets, jewellery and photo albums. You see it regularly as power spikes and brief black outs affect computers and lights even in the central business district.
Rob Hardy, Sydney, Australia

My family lives near Heathcote and nearly lost their business during last year's fires...hopefully it will again be saved. It's amazing what the firefighters are able to do with such limited resources. They are true heroes. However, it's a shame the government has been so ineffective to stop this nearly annual tragedy. One can only hope this is the final nail in John Howard's political coffin.
Jubilee Lang, USA

I live in Sydney and the smoke and ash from these fires dominates the landscape. Considering the vast area these fires cover it is a credit to the work of the fire crews that so few homes are lost.
Mike Inman, Australia

I've very recently been to Australia for a trip and I loved the country. I'm very sorry to hear that the beautiful city of Sydney is being threatened by these terrible fires, which might have been set off by some very stupid people!!!!
Klaartje Termeer, The Netherlands, Europe

The motorway has been closed Peter Jolly, Australia, Newcastle

What a nightmare developing. Roads are closed, houses in ashes, and people choking due to thick smoke. I live in Newcastle which is 2 hours north of Sydney. Thousands of commuters normally travel to Sydney for work, well that will not be happening today. The motorway has been closed, the rail line is shutdown and NSW is in fire crisis. Worst fires in 30 years!
Peter Jolly, Australia, Newcastle

The fires this time are amazing. One moment it was a normal (hot) summer day, and the next these huge walls of smoke sprang up everywhere.
Lincoln, Australia

I live in between the two main fires. On the first day of the fires I spent 45 minutes stranded on a train on the Harbour Bridge due to power disruptions. The fires over the last 12 hours have spread to larger forested areas and are worrying firefighters. To residents there is not much they can do apart from hope the fires are contained otherwise wait for rain. Australia needs to re-examine it's green policy which tries to preserve native vegetation
Hardeep Girn, Sydney, Australia

Although I am currently not in Australia, I've experienced quite a number of Australian bushfires in my life, most notably the 1994 ones and the recent ones that occurred on Christmas Day last year. In most (if not all) of these bushfires, arsonists have been the chief suspects of the bushfires.
John Markovina, Australia