First Union has expressed concern over what it sees as NZ First's growing influence in the Government.
The Government has been forced into some high profile policy back-downs from NZ First.
Labour's plan to repeal the three strikes law was canned because of objections from NZ First and more recently, Winston Peters has thrown plans to double the refugee quota up in the air.
First Union President Robert Reid said New Zealand First has got everything it wanted from the coalition agreement, especially on important issues like immigration.
"I guess we are a bit worried in the union... [about] the dominance on a number of issues that is coming from New Zealand First," he told The AM Show.
"It seems to be that everything that got tied down that New Zealand First wanted in the coalition agreement, Labour is going along with.
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"But if it wasn't in that agreement, the major party in some ways is being dictated to by the minor party simply because it requires those numbers to put things including industrial legislation, including migration [through]."
Newshub Political Editor Tova O'Brien said it was a bad sign for Labour that a union was voicing concern about the Government.
"We've been seeing these flashpoints between the governing partners but for one of Labour's bedfellows, the unions, to now be speaking out against [Labour]... this is next level," she said.
O'Brien said the next big hurdle for the Government would be the Employment Amendment Bill, where she predicts NZ First will not side with Labour.
"I think we'll start seeing NZ First siding with National on some of the amendments," she said.
The country won't see what goes on inside cabinet, but O'Brien thinks we will see some clear winners in the final versions of employment and migration legislation.
"When those issues do go to cabinet we're going to see the arm wrestle between Jacinda Ardern and Winston Peters," she said.
"It is going to be absolutely fascinating given this flagrant disregard by Winston Peters of Labour policies, it's going to be interesting to see who gets the first hand down onto the table."
Watch the full interview with Robert Reid above.
The AM Show with Duncan Garner, Amanda Gillies and Mark Richardson, weekdays 6-9am on RadioLIVE and streaming live to the rova app on Android and iPhone.