Isolating Transformer earth output

[Fuss]

My 230v Isolating Transformer has 150v on the earth output. Can anyone explain why this figure come about?.

The setup is as follows...
230v In, 230v out.
Input blue,brown earth connected to Inputs in transformer
Output blue,brown earth connected to Outputs in transformer

There is the option to tie down the output earth to the output neutral which solves the problem but my question is that when the earth is floating why the figure of 150v?