KiwiRail has admitted a fungus is rotting hard wood sleepers on the network, which appear to have contributed to two derailments.
Some 7000 of 100,000 sleepers imported from Peru in the past decade have been found in varying states of decay.
Many of them are on bridges or in tunnels.
Chief executive Jim Quinn says all sleepers showing signs of advanced decay have been replaced at a cost of between $250- $1000 each.
KiwiRail expects the replacement programme to be completed early next year.
New Zealand First says all the inadequate sleepers need to be replaced right away.
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