Robert Excell, 67, was freed from jail in Perth because of ill health. He emigrated to Australia aged 10 but never became a citizen.
He landed at Heathrow at 0700 BST. Excell was required to sign the sex offenders' register before being freed.
Children's campaigners have expressed fears he might re-offend in Britain.
Excell has spent 37 of the past 39 years in Australia's prison system for child sex offences dating back to 1965, when he raped a seven-year-old boy.--