The Council of Trade Unions (CTU) has closed its doors to the public and called the police after receiving a concerning response to the Christopher Luxon attack advertisement it released on Monday.
The advertisement calls Luxon "out of touch" and "too much risk". It features a picture of Luxon in a pinstripe suit looking forward sternly.
It upset the National Party, with campaign chair Chris Bishop releasing a statement and holding a press conference taking issue with how negative the election campaign was becoming.
On Tuesday, the CTU told Newshub most of the response to its election campaign had been "incredibly positive", but there had been "some mixed views".
"One communication in particular from an individual gave us some cause for concern," said Melissa Ansell-Bridges, the CTU General Secretary.
"As a result, we informed the police. Out of an abundance of caution, we have also closed the floors to public access for our Wellington office."
She said CTU would continue trying to ensure "New Zealanders understand what's at stake this election and we welcome responsible debate on the issues".
On Monday, NZCTU President Richard Wagstaff said the campaign was "focused on why a National-led government will leave working people worse off".
"Christopher Luxon and National will take New Zealand backwards and working people will be the first to feel the pain," Wagstaff said.
"The buck stops with Christopher Luxon. He's the leader, these are his policies. People need to take notice of that."
Wagstaff referenced a Newshub-Reid Research poll in May which found 47 percent of New Zealanders thought Luxon was out of touch compared with 35.6 percent who think Labour leader Chris Hipkins is.