Robbers hit Aeon's Tsz Wan Shan branch machine at 4am on Wednesday and its Sham Shui Po branch ATM at 5.20am yesterday. Thieves also tried to ransack Po Sang Bank's Causeway Bay machine at 5.46am.

All the targets were free-standing ATMs, which security experts say are more vulnerable than the 'hole-in-the-wall' type.

A third Aeon robbery, at Ting On Street in Ngau Tau Kok, in which $22,000 was taken, was reported at 3.20pm yesterday. An initial investigation showed the alarm did not work.

In the Sham Shui Po robbery, thieves entered the lobby of Aeon's Nam Cheong Street branch and levered open the machine to make off with a cash box containing $395,000, police said.

As with the first Aeon robbery, the alarm failed, said Sham Shui Po Senior Inspector Bernard Law Chiu-woon.

'They're very vulnerable; anyone can enter,' Mr Law said. 'The safe which was taken away has only two metal bars and a padlock. If you break the padlock, you can just prise a bar and take it.

'I think it's most likely copycats, unless there's a syndicate and they're employing a number of people.' Po Sang Bank assistant manager Cheung Wai-ki said security cameras showed a man opening the cover of its Pak Sha Road machine but he fled after spotting an alarm system.

'If they tried to open our safe the alarm would have activated,' Mr Cheung said.

Aeon managers failed to return calls. Securicor Hong Kong general manager Gilbert Fung Tak-choi said thieves appeared to have realised free-standing ATMs were easy targets.

A 15-year-old boy is due to appear in Kwun Tong Court today charged with robbing Aeon's Tsz Wan Shan machine.