KiwiRail has confirmed it will buy another 15 Chinese locomotives, despite reliability problems and asbestos contamination in the engines it already has.

KiwiRail has spent at least $12 million clearing the material from the engines, and documents show that 24 - at least half of them - that had been given the all-clear still contain asbestos.

Photo: SUPPLIED

Rail and Maritime Transport Union general secretary Wayne Butson said the DL class locomotives were unreliable and did not meet international performance benchmarks.

"The sad reality is the we've only just learnt in the last couple of days that the KiwiRail board have authorised the purchase of another 15.

"And I think everyone is aware that unless these locomotives become more reliable, then actually the future of the industry will be in jeopardy."

Mr Butson said the DL locomotives were not fit to be on the tracks.

However, a spokesperson for KiwiRail said they were confident the 15 new locomotives would be reliable and asbestos free.

They said KiwiRail needed the locomotives to meet customer demand, and they would come into service in 2018.

Forty of 48 Chinese-built DL-class engines have been part of massive clean-up effort since the problem was first discovered in February 2014.

Workers from China were brought in to remove the asbetos two years ago, but

KiwiRail has since discovered asbestos on the metal sheeting of the driver's compartment as a form of sound proofing.

It has also been found under paint, inside door panels and roof cavities.

The company invoked a warranty and the Chinese sent workers to New Zealand to repair the trains, with the clean-up costing $12 million as of February 2015.

However, this year the company found asbestos in the locomotives' gaskets as well.

The discovery was first revealed at an Employment Relations Authority hearing last month where the Rail and Maritime Transport Union was arguing Kiwirail should have used local workers for the clean-up.

Documents obtained by RNZ show the asbestos was discovered in April this year in the first and second generation DL Class locomotives, which were delivered in 2010 and 2013 respectively, many of which had already been through an asbestos removal process.

The contaminated gaskets are located in the exciter, a component which helps generate and regulate electricity produced by the alternator, and in an insulator gasket connected to the braking systems.

The latter was not notified to staff until 19 May.

The documents state KiwiRail will now remove 'Asbestos Free' stickers from many of the locomotives, and put 'Warning, Contains Asbestos' stickers on instead.

KiwiRail is yet to comment on whether those gaskets are being replaced with asbestos-free parts, but the documents said it was not likely to be a health risk unless disturbed.

Mr Butson said those gaskets were being removed.

He said ever since the first discovery they were finding "more bits of asbestos in various places".

"Who knows where we're going to continue to find it".

Mr Butson said the locomotives were unreliable, and on average failed every 20,000-30,000km compared to an international benchmark of 80,000km.

"These locomotives just have been an extremely bad purchase. They still can't operate at maximum line speed, the suspension's not good enough, otherwise they'll bounce off the track," he said.

"The DL locomotives we think are appropriately named - they're the dog and lemon locos."