A quarter of the 11.7tn yen (£91bn) the Japanese government allocated to rebuild the region devastated by last year's earthquake and tsunami has been spent on projects unrelated to the disaster.

A government audit also found about half of the reconstruction budget had yet to be distributed owing to red tape and indecision over how the affected communities should be rebuilt.

The revelations have prompted anger among survivors, who say reconstruction is taking too long.

More than 18 months after the disaster, about 325,000 people forced to flee the tsunami and radiation from the Fukushima Daiichi nuclear power plant are still living in temporary accommodation and have no idea when, or if, they will be able to return to their home towns.

A breakdown of expenditure for the 2011 portion of the disaster budget found a large amount had gone to projects that have little or no relationship to the reconstruction efforts in Iwate, Miyagi and Fukushima prefectures, where almost 20,000 people died on 11 March last year.

They included 500m yen for road construction in Okinawa, more than 1,000 miles to the south; 330m yen to repair a sports stadium in Tokyo; 10.7bn yen for a nuclear power research organisation; and subsidies for a contact lens factory.

Another 30m yen went to the justice ministry to buy equipment for prisons, while 2.3bn yen was given to the fisheries agency to protect Japan's whaling fleet from harassment by the marine conservation group, Sea Shepherd.

Other expenditure included renovations of government offices in Tokyo, training for aircraft and fighter pilots, research and production of rare-earth minerals, and semiconductor research.

The revelations have embarrassed the prime minister, Yoshihiko Noda, whose Democratic party of Japan won the 2009 election on promises to improve transparency and cut waste – particularly on public works projects.

"It is true that the government has not done enough and has not done it adequately," Noda said in a speech to parliament this week. "We must listen to those who say the reconstruction should be the first priority."

In a separate audit of a 9.2bn yen share of the total budget, Yoshimitsu Shiozaki, an expert in urban planning at Kobe University, found about a quarter had been allocated to programmes unrelated to the disaster.

"Legally speaking, there are no problems with these projects," Shiozaki was quoted as saying in the Japan Times. He said similar budget irregularities were found after the 1995 earthquake in Kobe, in which more than 6,000 people died, but added, "this time the funds are being used in a more deceptive way."

Government officials initially defended its use of disaster funds, arguing the cash would spark a general economic recovery from which the ravaged north-east would also benefit.

Officials said money had been granted to companies in different parts of the country because they had business relationships with smaller firms in the disaster region.

The trade and industry minister, Yukio Edano, insisted there was "no doubt" the distribution of cash had benefited communities affected by the tsunami.

Local media reported that money used to improve disaster readiness in areas untouched by the tsunami could be trimmed next year, but there is no sign yet that the government will rein in other dubious expenditures.

Noda acknowledged, however, that the revelations had angered the public, saying he would "wring out" spending on unrelated projects.

Reconstruction spending included in the budgets for 2011 and next year was supposed to help people affected by the nuclear crisis, rebuild infrastructure destroyed by the tsunami and create employment.

The government has pledged to spend 23tn yen by the end of the decade on reconstruction and disaster prevention, 19tn of it within five years.