Attempts to shift half a million trading accounts from ANZ Share Investing onto CMC's platform failed on Monday with many traders locked out of their accounts for hours.

Both ANZ and CMC acknowledge troubles with the migration, but would not provide a detailed explanation of what went wrong.

A spokesman for ANZ bank said the platform was up and running, but had experienced a significant increase in traffic.

"We understand how frustrating these situations can be for our customers and we apologise for the inconvenience caused,'' he said.