Foreign Minister Winston Peters is travelling to Washington DC for meetings with US Secretary of State Marco Rubio this week.
At home, Kiwi leaders have been watching Mr Trump's spiralling missives on social media with concern.
Mr Trump has threatened to attack civilian infrastructure in a bid to force an Iranian surrender, following US and Israeli attacks on the nation which began on February 28.
New Zealand Prime Minister Chris Luxon said the language was "unhelpful" and "more military action is not necessary".
"We're gravely concerned," he told Radio NZ.
"We expect all parties to comply with international law.
"Threats from the president over the weekend ... bombing bridges and reservoirs and civilian infrastructure, would be unacceptable.
"Our very consistent message has been to encourage the US and Iran to de-escalate quickly."
Opposition Leader Chris Hipkins went further, saying Mr Trump's behaviour was "alarming", "totally unacceptable" and the president was "diminishing" his office.
"It's certainly not the behaviour that we would expect to see of somebody in the powerful position that Donald Trump is in," he said.
New Zealand's two major parties have split on the war, with Mr Luxon's conservative government non-committal in its stance towards the US-Israel action.
Mr Hipkins' Labour opposes the operation as illegal and likened it to the 2003 Iraq invasion, which New Zealand also condemned on similar grounds.
Both major parties in Australia, in contrast, have supported the US-Israel action in Iran.
The forthright language from New Zealand also stands in opposition to major party politicians in Australia, who have generally been less critical of the US president.
Unlike Australia, New Zealand is not formally allied to the United States, although it is strategically aligned with both Canberra and Washington on curbing China's rise.
Mr Peters, who is currently travelling to the US capital, says this week's talks are critical and come in the "most challenging global context" since WWII.
In an interview with AAP before the Iran crisis, he said he intended to use the meeting to attempt to re-engage US interest in the Pacific.
Mr Luxon confirmed that issue would still shape the bilateral talks, along with information-gathering over US plans for Iran.
The primary domestic concern for New Zealand from the Middle East attack is fuel shock.
Average petrol prices have leapt to $NZ3.48 ($A2.91) and diesel to $NZ3.75 ($A3.10), prompting government action to secure supplies and build deeper reserves.
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