The ABC has issued a stern email to staff, warning employees - especially high-profile radio and television personalities - against voicing their opinions about marriage equality.

The memo comes after Lateline host Emma Alberici called Australia's handling of same-sex marriage an "indictment" on both sides of politics on social media.

In an all staff email, the ABC's editorial policy manager Mark Maley said approximately 40 per cent of Australians oppose changing the country's marriage laws. He also said the national broadcaster "does not have a position on the issue".

"It is very important that we are impartial and that all perspectives are given a fair hearing and treated with respect by the ABC," he said.