Dominion Diamond Mines could lay off 150 "entry-level workers" at the territory's Ekati mine over the next four months.
In a message to members posted on its website, the Union of Norther Workers said Dominion had informed it of the move late last week.
Dominion says it's considering action because workers simply aren't turning up, and the consequent economic impact is damaging the company.
The union told its members: "The employer’s intent is to replace these positions with contracted workers by
October 1, 2018 as a cost-cutting mechanism to increase profitability for the private owner.
"The UNW/PSAC [Public Service Alliance of Canada] will be taking swift action by filing grievances and unfair labour practices complaints with the Canada Industrial Relations Board, as well as looking at several other options."
Dominion issued a response on behalf of chief executive Patrick Evans in which the company said: “Last week, Dominion Diamond Mines gave notice to the President of the Union of Northern Workers that the company is considering contracting out certain mine operations support positions due to unacceptably high levels of absenteeism that have negatively impacted the costs of these operations.
"Pending a final decision, we have invited the Union to put forward proposals to address this serious and unsustainable situation.
"We are committed to considering any proposals put forward by the union. However, if we cannot resolve issues through discussions with the union, the contemplated contracting will proceed."
Dominion operates the Ekati mine and also holds an interest in the neighbouring Diavik mine, but is not that mine's operator.
Last week, Dominion and its parent, the Washington Group of Companies, announced preparatory work on a major expansion at Ekati – the Jay pipe – would be halted while executives assess their options.
The move suggested Dominion is no longer sure the Jay expansion will be feasible, given the poor performance of diamond prices and the significant cost of constructing a dike to reach the underwater Jay deposit.
At the time, Dominion did not provide any guidance as to the impact of that suspension on staffing levels at the mine.