Northland-based Indonesians are mourning the toll from the country's deadly tsunami, but also breathing a sigh of relief their family and friends back home aren't affected by the killer wave.

The tsunami has killed more than 420 people and left thousands homeless. More than 500 buildings have been destroyed or heavily damaged as waves poured in from the Sunda Strait, which separates Indonesia's two most populated islands of Java and Sumatra.

Authorities warned the death toll could rise as they widened searches and treated nearly 850 people injured by the three-foot surge of water off the western tip of Java island about 96km from Jakarta.

The tsunami slammed ashore near the erupting Anak Krakatau volcano and swept over fishing ports and hundreds of people at a holiday beach concert without warning late on Saturday.

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Indonesian Eva Corne, who has lived in Whangārei for 18 years and teaches belly dancing, said she had yet to speak to her family who lived in Jakarta but presumed they were safe as she hadn't heard anything to the contrary.

Her two brothers live in Jakarta and another in Bandung.

"I am not sure whether they felt the impact of the tsunami but they live in the ring of fire and so are really prone to tsunamis and earthquakes. Luckily, they are safe.

"It will take a long time for those affected to recover and then there are little places where rescue teams haven't arrived yet. But the response from the government has been speedy."

Corne helped Friends of Indonesia organise a variety concert in Onerahi last month to raise money for victims of a powerful earthquake in Sulawesi that killed more than 2000 people and forced more than 82,000 from their homes in September.

Before that, she collected much-needed funds for the victims of an earthquake on the island of Lombok in August.

"I first heard about the tsunami on Sunday morning and thought 'not again' after what happened on Lombok and in Sulawesi. I was planning to go to Jakarta next month but am not sure now."

Another Northland-based Indonesian, Indri Donaldson, used to work for an Australian mining company off the western tip of Java four decades ago and praised the resilience of the local people.

She, too, has family and friends in Jakarta who were not affected but who are concerned about those still missing.

"It happened at night without any warning but people there are family-oriented and with help from their neighbours Australia and New Zealand, they will pick up the pieces and move on."

There are 15 Indonesian families living in Northland, the majority in Whangārei and the rest in Kerikeri, Paihia, Kaitaia and Russell.

Indonesian seismologists initially speculated that a landslide - possibly undersea - may have generated the tsunami.

The event did not create tremors that normally put coastal residents on watch for tsunamis. Officials also issued no formal tsunami warnings.