At least seven people died after an Indonesian police station was targeted in another Islamist bomb attack the day after 13 Christians were killed in suicide bombings aimed at churches.

Another suicide bombing ripped through Surabaya, Indonesia's second-largest city, shortly before noon on Monday, prompting a warning for thousands of Australians holidaying in Indonesia to 'exercise a high degree of caution' and consider leaving.

Closed-circuit TV footage showed four men, linked to a pro-ISIS extremist group, riding on two motorcycles towards police officers who stopped them at the gate.

It was the latest suspected Islamic terrorist attack to coincide with the holy Muslim month of Ramadan.

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At least seven people were killed outside Surabaya police station on Monday in an attack (pictured) that came the day after thirteen were killed in suicide bombings

This is the family who committed the church suicide bombings on Sunday

CCTV captured the moment the bomb detonated outside the station about 11.50am Monday

After a brief pause, the bomb detonated, with a giant explosion sending several people hurtling to the ground in a shower of debris.

The blast in the Javanese city killed at least seven people with the death toll still to be determined.

Police said four officers and six civilians were injured and suspected it was carried out by Islamist extremists linked to Jemaah Ansurat Daulah, which was formed in 2015 by ISIS sympathisers.

'Clearly it's a suicide bombing,' East Java police spokesman Frans Barung Mangera told a media briefing on Monday which the ABC attended.

'We can't be open [about] all details yet because we are still identifying victims at the scene and the crime scene is being handled.'

The 3,224 Australians now holidaying in Indonesia, mainly in Bali, have been warned of a heightened risk of terrorist attacks after a spate of suicide bombings ripped through Christian churches and police headquarters in Surabaya.

The East Javanese city, Indonesia's biggest metropolis outside Jakarta, was reeling after a car bomb exploded at a police headquarters on Monday, after a family of six blew themselves up on Sunday, killing 13 and wounding 40 Christians attending services at three churches.

The Muslim fasting month of Ramadan begins on Tuesday, and the Department of Foreign Affairs and Trade's travel advisory urged people to 'pay close attention to your personal security at all times'.

'Exercise a high degree of caution in Indonesia, including in Bali, Surabaya and Jakarta, because of the high threat of terrorist attack,' it said on its Smart Traveller website on Monday.

'Australians are reminded of the heightened risk of further attacks in the lead-up to and during Ramadan (mid-May to mid-June 2018).'

The explosion killed at least seven people, although the full extent of casualties remains unclear. (Officers pictured standing guard at police headquarters in Surabaya)

More than one million Australians travel to Indonesia every year, mainly to the popular resort island of Bali, where Australians were killed in terrorist attacks in 2002 and 2005.

An estimated 10,000 Australians live in Bali alone all-year round.

Indonesia is also the second most popular overseas holiday destination for Australians, after New Zealand, with more than 1.2 million Australians visiting its northern neighbour in 2016.

Australian Prime Minister Malcolm Turnbull condemned the attacks as an act of 'barbarity' and expressed his condolences to Indonesia, the world's largest Muslim-majority nation.

'Our heart goes out to all of the victims of those attacks and the people of Indonesia know that they have our utmost solidarity in the battle against terrorism,' he told reporters in Sydney on Monday.

'It almost beggar's belief; the brutality, the barbarity, the inhumanity, the blasphemy of these terrorists.'

Prime Minister Malcolm Turnbull (pictured centre) condemned the attacks as an act of 'barbarity' and expressed his condolences to Indonesia, the world's largest Muslim-majority nation

Home Affairs Minister Peter Dutton said it was an act of evil.

'This threat is not going away and Australia stands absolutely united with a very, very crucial friend in Indonesia to make sure we can work with the president and ... the Indonesian government to keep their people safe,' Mr Dutton told reporters in Canberra on Monday.

'Evil has prevailed in recent hours.'

A day earlier, suicide bombers from the same family killed at least 13 people and wounded 41 in attacks on Christians attending Sunday morning services at three churches.

All six family members were killed in the attacks and at least 41 people were injured, in acts that Indonesia's president condemned as 'barbaric'.

A day earlier, suicide bombers from the same family killed at least 13 people and wounded 41 in attacks on Christians attending Sunday morning services at three churches

All six family members were killed in the attacks and at least 41 people were injured. Pictured: Officers load a body bag into an ambulance at the site of an attack

The bombings were the worst to target churches in Indonesia since a series of attacks on Christmas Eve in 2000 killed 15 people and wounded nearly 100.

National police chief Tito Karnavian said that the father exploded a car bomb, two sons aged 18 and 16 used a motorcycle for their attack, and the mother was with daughters aged 12 and nine for her attack.

Mr Karnavian said the family had returned to Indonesia from Syria, where until recently, the Islamic State group controlled significant territory.

The extremist group claimed responsibility for Sunday's attacks in a statement carried by its Aamaq news agency.

President Joko 'Jokowi' Widodo (centre left) visited the scenes of the attacks and described them as 'cowardly actions' that were 'very barbaric and beyond the limit of humanity'

The bombings were the worst to target churches in Indonesia since a series of attacks on Christmas Eve in 2000 killed 15 people and wounded nearly 100. Pictured: Officers remove wreckages of burnt out motorcycles

Mr Widodo with officials at the scene of a suicide attack outside the Central Pantekosta church in Surabaya

The first attack struck the Santa Maria Roman Catholic Church in Surabaya, police spokesman Frans Barung Mangera told reporters.

That blast was followed by a second explosion minutes later at the Christian Church of Diponegoro and a third at the city's Pantekosta Church.

Three unexploded homemade bombs - two at the Pantekosta church and one at the Diponegoro church - were detonated by a bomb squad.

President Joko 'Jokowi' Widodo visited the scenes of the attacks and described them as 'cowardly actions' that were 'very barbaric and beyond the limit of humanity'.

A fourth attack occurred on Sunday evening outside the gates of nearby Taman Police station in Sidoarjo, East Java.

A police spokesperson confirmed a bomb was detonated, although there have been no reports of casualties since the blast.

Former prime minister Kevin Rudd said the terrorist attacks were a 'sobering reminder' of the new danger posed by Islamic State fighters returning from Syria and Iraq.

'Significant threat to Australians and westerners in SEA (South East Asia) region,' he tweeted.

Three unexploded homemade bombs - two at the Pantekosta church and one at the Diponegoro church - were detonated by a bomb squad. Pictured: Anti-terror policemen stand guard following one of the attacks