A man who targeted elderly women over a month-long crime spree in Auckland has been locked up indefinitely.

William Murphy has been sentenced at the High Court in Auckland on 15 charges, the most serious of which is aggravated robbery.

However he has an horrific rap sheet of previous sex and dishonesty convictions.

That's prompted Justice Mark Woolford to impose a sentence of preventive detention - with a minimum non-parole period of six years.

Murphy could spend the rest of his life in prison, if the Parole Board believes he continues to pose a threat to the community.

An elderly woman said she hasn't recovered from being robbed by Murphy, and will leave Auckland.

The Pakuranga victim said she's lived with pain since being attacked.

She's relieved Murphy has been indefinitely jailed after he targeted her and other elderly woman in a month-long crime spree.

The High Court at Auckland heard Murphy forced his way into the woman's unit at a retirement village, punched her in the face several times and forced her to take off her trousers and underwear.

After rifling through her belongings, he found credit cards and demanded she told him her pin numbers, holding a knife to her throat.

His sentence is officially six years but he has a long rap sheet of sex and dishonesty crimes, and the Parole Board could keep him in jail if it believes he continues to pose a threat to the community.