A Victorian union boss says he is dismayed and disgusted by the state Labor government's promotion of construction company Grocon.

In a letter to Premier Daniel Andrews, Construction division secretary John Setka from the Construction Forestry Mining and Energy Union threatened to pull support for the government, accusing ministers of “flaunting” their relationships with Grocon.

Over the weekend, Police Minister Lisa Neville tweeted thanks to Grocon for work on the Wye River, Separation Creek fire recovery – prompting Mr Setka to tweet back: “Disgraceful coming from a Police Minister and an absolute insult to victims of crime. Should stand down now.”

In his letter on Monday, Mr Setka said building unions had been in an “industrial war” with Grocon for years.

He pointed to the deaths of four people at Grocon sites, including three deaths that the company pleaded guilty to charges over and were fined.

“The Victorian branch of the CFMEU Construction and General Division, on behalf of its members, cannot and will not continue to support your government if it continues with the pathetically weak and morally corrupt stance of promoting and supporting companies such as Grocon,” Mr Setka wrote.

On Tuesday, Mr Andrews would not comment on the letter.

“I get letters and representations from hundreds and hundreds of different groups across the community and just like all of them, the CFMEU are entitled to their view,” he told reporters.

Opposition leader Matthew Guy said the letter “shows you who’s in charge of this government”.

Grocon has been approached for comment.