Train passengers on the troubled Wairarapa line were left sweating twice last week, with in-carriage temperatures soaring to nearly 40 degrees Celsius on Thursday.

Train operator Transdev said the incident, which followed the air conditioning system breaking down two days earlier, came as a result of the aircon being turned on later than usual.

Commuter Greg Kerr recorded a temperature of nearly 40C on a cellphone app.

He said the carriage was so hot he could feel the heat before stepping on board.

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"It's absolutely unacceptable," he said.

"I got on the train in Petone, and I could feel it before I even got in the cabin. Nothing was said until we asked what was going on later in the Hutt."

Kerr said water was handed out by a conductor, although it was tepid by the time passengers received it.

Further adding to the misery, he said the toilets in his carriage were full and "swishing around the place".

The incident came just days after a fellow commuter jokingly threatened to strip off his suit in favour of speedos, to protest against ongoing issues on the train.

Transdev customer and communications manager Emily Liddell said the heat stemmed from maintenance on the train's generator carriage.

"The air conditioning system was working, but is usually turned on much earlier in the day to cool the carriages before passengers board the service," she said.

"Essential maintenance is required on all carriages on a regular basis, in this case it was a required oil change and needed to be undertaken at this time. Unfortunately, the maintenance on this carriage coincided with a particularly hot day in Wellington.

"There were no safety concerns with the temperature in the carriages, but we accept it may not have been comfortable for customers.

"We apologise for the warmer temperatures on this service yesterday and can reassure passengers that we are endeavouring to ensure that this doesn't happen again."