Australian Opposition Leader Bill Shorten has gone public for the first time about "untrue and abhorrent" allegations he raped a teenage girl almost 30 years ago.

Victoria Police have decided not to lay criminal charges after advice from prosecutors that there was was no reasonable prospect of conviction.

The allegations apparently relate to events at a Young Labor camp near Geelong in 1986, when Shorten was 19. The complainant, now a nurse, was 16 at the time and Shorten said he knew her briefly.

She first made the allegation on the Facebook page of former Prime Minister Kevin Rudd after Shorten became Opposition Leader in 2013. That was when Shorten first became aware of the allegation for which there was "absolutely no basis".

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"I will not go into the details except to say that the allegation was untrue and abhorrent," he told reporters in Melbourne on last night.

Shorten said the matter had been vigorously investigated by police and he co-operated to clear his name. "The easy option would be to say nothing but that is not who I am." Shorten said the matter had been distressing and he was thankful for the support of his wife Chloe and the backing of staff and colleagues.

- AAP