At least one of the two Bondi Beach gunmen was known to authorities before the worst shooting massacre in Australia in almost 30 years.

At least 12 people were killed when the pair opened fire on a Jewish event being held at the beach for the first day of Hanukkah on Sunday.

One of the gunmen was killed while the other was arrested. Here is what we know so far:

At least one of the two Bondi Beach gunmen was known to authorities before the attack. (Nine)

Who are the gunmen?

The identities of the gunmen have not yet been made public.

One of the shooters was known to authorities but had not been considered an immediate threat, both NSW Police Commissioner Mal Lanyon and ASIO Director-General Mike Burgess said.

It was not known what hospital the surviving gunman had been taken to, but police said he was in a critical condition.

Is anyone else involved in the attack?

There had been reports that there may have been a third shooter, but police eventually clarified that there were only two gunmen.

Burgess said ASIO was urgently investigating anyone who knew the attackers and looking at whether there was “anyone who has similar intent”.

“It’s important to stress at this point that we have no indications to that fact, but that is something we have active investigations on,” Burgess said in a press conference.

Police at a home in Bonnyrigg, in Sydney’s west, after the Bondi Beach shooting. (9News)

Where are police investigating?

Officers swarmed on a property in Bonnyrigg in Sydney’s west in the hours immediately after the shooting.

The property was believed to be the home of one of the gunmen.

Streets were closed off as tactical teams and detectives surrounded the scene.

Large areas of Bondi Beach remained a crime scene through the night as forensic police gathered evidence.

The bomb squad was also called in after suspected improvised explosive devices were found in a car at the beach.

Those devices were removed and the exclusion zone was scaled back but the area remained a crime scene.

Heavily armed police at Bondi Beach. (Darrian Traynor/Getty Images)

Is there an ongoing threat?

Burgess said the national terrorism threat level remained as “probable”.

“I don’t see that changing at this stage,” he said on Sunday night.

“‘Probable’ means there is a 50 per cent chance of an act of terror and unfortunately we have seen that horrific act occur in Australia.”

Who is investigating?

NSW Police declared a terrorist incident, and those officers will be supported by Australian Federal Police (AFP) and the Australian Security Intelligence Organisation (ASIO).

Prime Minister Anthony Albanese, ASIO Director-General of Security Mike Burgess and AFP Acting Deputy Commissioner Nigel Ryan address the media in the hours after the Bondi attack. (Alex Ellinghausen)

Prime Minister Anthony Albanese confirmed the National Security Committee had met on Sunday night.