A moped and a car were involved in the crash.

A person has been seriously injured in a crash on Karangahape Rd in Auckland's CBD - forcing its partial closure.

Police said the crash happened on the K-Rd overbridge, between Pitt St and Howe St before 1.15pm on Wednesday.

A witness described seeing a black car in the middle of the glass-covered road, and a mangled moped near by.

Auckland Transport The incident happened on K-Rd in Auckland's CDB

"One person is in a serious condition and has been taken to hospital," a police spokesman said.

Only east-bound traffic heading towards Grafton Bridge was allowed through.

West-bound lanes, including those heading towards Ponsonby, were closed to allow emergency services to attend the incident.

ABIGAIL DOUGHERTY/STUFF The Serious Crash Unit is investigating the collision.

Auckland Transport urged motorists to avoid the area or follow diversions.

Due to a serious crash westbound lanes on a section of Karangahape Rd, between Pitt St & Howe St in the CBD, are now CLOSED. Eastbound lanes remain open currently. Please avoid this area or follow the directions of emergency services for diversions. ^TP pic.twitter.com/WFTfk75pm4 — Auckland Transport (@AklTransport) June 13, 2018

The Serious Crash Unit was attending.