UPDATED 3.22pm: A person has died after falling from a neighbouring building on to Newmarket train tracks this afternoon.

A police spokesman said officers arrived about 12.15pm and were still examining the scene of the fatality.

It appeared the deceased person had fallen off a neighbouring building and landed on the tracks, the spokesman said.

Auckland Transport said a train shuttle was operating between Onehunga and Penrose.

Passengers travelling to stations on the Southern Line between Otahuhu and Newmarket should use scheduled buses from Great South Rd, it said.

A St John spokeswoman said two vehicles attended the incident about 12.15pm.

One patient with critical injuries was treated but no transport was required.

No further details could be released, she said.

A Fire Service spokesman said a unit had attended.

Auckland Transport told passengers the "emergency services incident" had closed the station about 12.30pm.

It tweeted that services on the Onehunga Line were suspended because of the incident, while Southern Line services were operating via the Eastern Line.

Western line services were by-passing Newmarket, it said.

The delays come on the first day of the electric train roll-out, with Auckland Transport replacing its remaining old diesel trains with electric trains on all lines from Papakura and Swanson to Britomart today.