You need an RF attenuator. Something like
this would be ideal 'cause it has variable attenuation. Not sure if Dick Smith sell them, though.
As an alternative you could try putting a passive spitter in the line - something like
this. It will lower the RF power by around 3dB, which may be enough to fix your problems.
The other solution is have someone lower the gain on your masthead amp. You could do it yourself if you know what you're doing. Otherwise get someone in to do it.