A woman claiming to be Kate Middleton sped the wrong way down a one-way street this morning before rolling her car on State Highway 45 in Taranaki.

Police Senior Constable Grant Kennard said the 44-year-old woman was mentally unwell and had refused to let emergency services help her after she crashed about 7am.

"She thinks she's Kate Middleton. She was saying: 'You can't touch me because I'm royal'."

Earlier this morning the woman had been reported driving more than 80km the wrong way along Courtenay St in New Plymouth.

"It's lucky there was nobody coming the other way."

Before the woman crashed near Tataraimaka on SH45 between Oakura and Okato she had been weaving across the road, Kennard said.

"A car had to swerve to miss her. It was a close shave."

Her car landed on its roof after hitting a bank and rolling.

The woman was not injured.

"She's obviously someone who is not in a fit state of mind to be driving."

Kennard said the woman was being assessed by mental health professionals and it was unknown if charges would be laid.