Eight children ranging from babies to teenagers were found dead at a home in the Australian city of Cairns Friday, police said, reportedly in a gruesome mass stabbing just days after a deadly siege in Sydney.

Prime Minister Tony Abbott called the Cairns tragedy heartbreaking, with Australia already in an outpouring of national grief over Monday's dramatic standoff in Sydney, which left two hostages and the gunman dead.

Australian police late Thursday searched several properties across Sydney, reportedly as part of a ongoing counter-terrorism investigation but not related to the deadly cafe siege earlier this week, officials said.

New South Wales police said the raids were unconnected to the 16-hour standoff at a Sydney cafe on Monday that left the lone gunman, self-styled Islamic cleric Man Haron Monis, and two hostages dead, but would not provide further details about the operation.

Four homes were destroyed as Australian firefighters struggled to bring two major blazes under control in Victoria state with high temperatures and gusty winds ahead of a cool change later Thursday.

Lightning strikes this week ignited 350 fires with several becoming significant, according to authorities. Some 500 firefighters are tackling the outbreaks with nearly 10,000 hectares (24,700 acres) razed.

Australian Prime Minister Tony Abbott on Thursday admitted Sydney's cafe siege was "an horrific wake-up call" as details emerged of the final minutes of the standoff which left the gunman and two hostages dead.

Abbott has ordered an urgent enquiry into the circumstances surrounding the tragedy and why deranged self-styled Islamic cleric Man Haron Monis was not under surveillance given his history of extremism and violence.

Iran repeatedly warned Australia about the criminal past of the perpetrator of the Sydney cafe siege and called for him to be kept under surveillance, top officials in Tehran said.

Man Haron Monis, the Iranian-born self-styled cleric who died along with two of the people he had taken hostage, was being investigated over fraud charges when he fled in 1996, police said. 

Australian Prime Minister Tony Abbott on Wednesday ordered an urgent inquiry into why a deranged Islamic gunman was not under surveillance and how he obtained citizenship, as security was stepped up after a fatal cafe siege.

Iranian-born Man Haron Monis, 50, had a history of extremism and violence and was on bail for a string of charges, including sexual offences and abetting the murder of his ex-wife.

Iran's media reacted Tuesday to the deadly hostage siege in Sydney by pointing the finger of blame at the West and linking the Iranian-born gunman to Islamic State group militants.

One of the bouquets left on Tuesday in a sea of flowers at the site of the Sydney siege read "I will ride with you." It was as much a wholehearted message to Australia's Muslim community as to the incident's victims.

Those words are the theme of a Twitter campaign of solidarity, offering to accompany any Muslims who fear venturing onto public transport at this time, in case of a public backlash after Monday's deadly cafe siege by an Iranian-born extremist. 

Australia's government pledged Tuesday to determine why an Iranian-born Islamist with a history of extremism and violence was able to play out a "sick fantasy" by seizing hostages in a day-long siege.

A giant sea of flowers was laid at a makeshift memorial near the scene in the heart of Sydney's financial quarter, where Muslim community leaders joined their fellow citizens in mourning the two victims of Monday's cafe siege.

Two hostages and the lone Iranian-born gunman were killed as heavily armed Australian police early Tuesday dramatically stormed a central Sydney cafe to end a day-long siege sparked when the "self-styled sheikh" took 17 people hostage.