The arch-conservative South Australian senator Cory Bernardi has been under fire from moderates in his party for extending an invitation to visit Australia to the Dutch politician Geert Wilders.

Mr Wilders, whose party holds the balance of power in the Dutch parliament, is Europe's most famously anti-Islamic politician. He has likened the Koran to Mein Kampf and Islam to fascism.

Yesterday more evidence of Senator Bernardi's anti-Islamic views were revealed when Australia's largest online Muslim forum, Muslim Village, published a letter from the senator to a Sydney Muslim.

Senator Bernardi was responding to concerns expressed by the man about his negative comments regarding Islam.

In the letter Senator Bernardi declared Islam had been linked with hate speech, terrorism, gang rapes, racism, segregation and isolationism and even false reports of assault to NSW police.

Then Senator Bernardi stated: "You have identified yourself as an Australian Muslim. I would be interested to know if you subscribe to fundamentalist Islamic practices.

''If not, I ask then for evidence that you have publicly denounced the above-mentioned practices and the preachers who advocate for non-engagement of Muslims and 'infidels'."

The Sydney man replied and listed extremism associated with other religions. On August 12, Senator Bernardi wrote a letter stating the man was ''incapable of critically analysing the actions of Islamic fundamentalists''.

''In the absence of your condemnation, one can only conclude you agree with their conduct.''

The letters will only add fuel to the fire burning in the Liberal Party regarding Senator Bernardi's increasingly vituperative anti-Islamic comments.

The issue has also given Labor, struggling with its own problems, a chance to direct media scrutiny to the opposition.

The man who would have the final say on letting Mr Wilders into Australia, the Immigration Minister, Chris Bowen, criticised Mr Wilders and Senator Bernardi on ABC television yesterday.

''Mr Bernardi is a serial offender when it comes to matters of inappropriate commentary on other people's religion and he is Tony Abbott's right-hand man … clearly his frontbench position and his position as a key adviser to Tony Abbott really needs to be brought into question.''

The opposition treasury spokesman, Joe Hockey, criticised Mr Wilders.

''Neither Cory Bernardi nor the Coalition will be facilitating this visit,'' he said.

''This gentleman seems like a very unpleasant character and I think it's far better that we have nothing to do with him,'' he said.