"This is a historic day that our members worked very hard for", said Jerry Dias, Unifor National President.
"British Columbians have voted overwhelmingly to replace Christy Clark's Liberals with a new government that works better for families".
Weaver stressed that the deal is not to create a coalition government, but for the Greens to prop up the NDP as the governing party. "We also had good negotiations with the B.C. Liberals, and what was obvious to us, the B.C. Greens, is that there is so much commonality in this legislature", Weaver said.
Unifor is Canada's largest union in the private sector, representing more than 310,000 workers.
Weaver said it's important to demonstrate a minority government can operate stably for a full four-year term.
The deal would give them a combined one-seat majority in the legislature, bringing down the Liberals who have ruled the province for 16 years.
Weaver said the top priority of the Green caucus will be to "collaborate across party lines with both other parties to advance good public policy that puts the people of British Columbia first".
The Greens, with just three seats, hold the balance of power.
The Greens' longstanding opposition to new and expanded oil pipelines has been well-documented, and that stance appears to have played an integral role in their negotiations with the NDP, who have also opposed projects like Kinder Morgan's Trans Mountain expansion, which has already been approved by the federal Liberals.
UK: Manchester attack investigation still at 'full tilt'
Grande, meanwhile, promised to return to "the incredibly fearless city of Manchester " to hold a benefit concert for the victims. She went on to defend the work of the security services, amid reports that warnings about Abedi were not followed up.
The Greens have made it clear that, in exchange for their support, they want support for their democratic reform initiatives.
"That includes Kinder Morgan and making sure we have access to tidewater for our products", said Alberta Deputy Premier Sarah Hoffman.
The Greens had given themselves a self-imposed deadline of Wednesday to decide which party to back after holding talks with both the Liberals and the NDP.
Green leader Andrew Weaver and NDP leader John Horgan at weekend rugby game. The left-leaning NDP won 41 seats and the Green Party won three.
A deal between the two parties could dismantle Premier Christy Clark's Liberal government as the Liberals failed to get a majority during the election.
"I'm very excited about the prospect of delivering the people of British Columbia what they voted for, and that was change", NDP leader John Horgan told reporters at the legislature.
But in hindsight, Clark's message looked more like an attempt to get out in front of the Horgan-Weaver announcement.
After all, he said it would only take one New Demcocrat to miss the ferry to Vancouver Island for the vote in the legislature in Victoria and "the Liberals could survive a confidence vote on the throne speech". "This issue of Kinder Morgan is one that was critical to us, and I think you'll see that reflected in tomorrow's announcement", he said.
On Monday, Clark posted a video on her Facebook page, saying she knows there's a lot of uncertainty about who would form the government. Next morning, Weaver cancelled a scheduled follow-up session and made a decision to go with the more substantive offer from the NDP instead.