Initiatives such as scenic carriages, a new menu and packages including whale watching and wine tasting have been introduced since 2011 to try and boost passenger numbers.
"However, we still have a business to run and the revenue loss from the drop in passenger numbers is not sustainable," said Ms Hume.
Rail and Maritime Transport Union (RMTU) General Secretary Wayne Butson challenged the decision, saying suspending the service would reduce much-needed tourists to the region.
"To suspend the service over the winter months is short-sighted - it sends a signal to tourism operators overseas that our own Government lacks faith in the rebuild of Christchurch and the resurgence in tourism in the South Island and beyond.
"There are plenty of businesses in the South Island who benefit from the tourist dollars that services like the TranzCoastal bring in," he said.
The Coastal Pacific service is operating for an extended summer between October and April.
The service will close for winter from May 5 and resume again at the start of October.
All tourism operators in and out of Christchurch had had a particularly hard time over the last couple of years, Labour's tourism spokesman Andrew Little said.
"It's recognised by everyone that it will be a slow build to recover tourist traffic but suspending services like the Coastal Pacific link will only add to the problem,'' he said.
The party's transport spokesman Iain Lees-Galloway said KiwiRail's decision reflected the extreme and unrealistic pressure placed on it to quickly turn around years of underinvestment under private sector ownership.
"If the Government's expectations of KiwiRail were more realistic and it took a more hands-on approach to economic development, the Coastal Pacific service - and others that are being cut back around the country - might have a fighting chance.''
