In a joint statement, six aid groups said the scale of the devastation left by the 6.2 magnitude quake on May 27 was far greater than had been initially estimated, particularly in terms of the numbers left homeless or destitute. As many as 1.5 million people are sleeping in makeshift shelters or tent cities around the quake zone. Some are too frightened to return to those houses that are still standing, fearing further aftershocks.
On June 1, the UN appealed to members for $103m (£56m) to cover a six-month emergency response, but until now only $21m has been pledged. Indonesia has estimated that the total damage caused by the earthquake could reach $3bn, including lost economic activity, of which around half would be the cost of replacement housing.
Aid groups trying to raise money for emergency care say they are running short, holding up efforts to provide shelter and sanitation for vulnerable survivors. Oxfam, which is seeking to raise around £3m for a three-month relief programme, says it is still £2m short of its target.
"People are sleeping outside in unclean conditions, which means disease is already spreading frighteningly fast in some villages. We're doing everything we can, but if relief operations don't get the support needed to help all the survivors soon, things will get worse," said Johan Kieft, an emergency response official at Care International.
The official death toll from the quake that struck south of the city of Yogyakarta is 5,736 people, with nearly 80,000 injured. It left its mark on a zone that measures roughly 193 square miles.
But while the human toll in Java is far lower than that of the catastrophic Indian Ocean tsunami in 2004 or the 2005 Pakistan earthquake, the number of people left homeless is emerging as a huge challenge.
The Indonesian government estimates that over 150,000 houses were completely destroyed and a further 184,000 sustained heavy damage. Almost all were family homes, so the total number of homeless may be as many as 1.5 million. This would be three times as many as the number left without accommodation in Aceh province, which bore the brunt of the 2004 earthquake and subsequent tsunami.
The UN initially estimated that between 100,000 and 200,000 people in Java would need housing, but aid workers say the real number of homeless has soared as the scale of the disaster has sunk in.
Java is much more densely populated than Aceh and its housing stock offered little resistance to last month's giant tremors, said Oxfam spokesperson Harriet Binet. "It does appear that the housing structures here were fairly weak, so a lot of homes just crumbled after the quake. That's why there's such a huge number of destroyed or damaged homes," she told the Guardian. "Our concern is emergency relief. There's still a need for more shelter supplies, and water and sanitation work."
Indonesia has promised to pay around $3,000 in compensation for each house destroyed by the quake.