Eighty-two-year-old conservative Christian MP the Reverend Fred Nile has been called many things in his time in public life, but last week was the first time he was deemed a threat to international security.

Mr Nile MLC, the longest-serving member of the NSW Parliament, says both he and his wife, Silvana Nero, were last week denied entry to the United States because they were told they presented an unspecified security risk.

Mr Nile said he had booked travel to the US after being invited to attend the inauguration of President Donald Trump on January 20.

"No explanations or reasons were given," said Mr Nile, the founder of the Christian Democratic Party.