National's police spokesman Mark Mitchell said the dousing of controversial activist Posie Parker with tomato juice was assault.
The incident happened on Saturday, when transgender rights supporter Eliana Rubashkyn passed a cordon masquerading as a Parker supporter and tipped the juice on her.
Today, Mitchell said that was an assault.
"Make no mistake. Can you imagine if any of us walked up to someone else and started pouring fluid — by the way [Parker] would have had no idea what it was — that is traumatic.
"We shouldn't accept it and it is absolutely an assault."
Mitchell said if someone was assaulted while trying to exercise their freedom of speech then there'd "been a failure".
"It sends a terrible message because in this country we should really defend and protect our freedom of speech — whether you agree with the person and what they're saying is irrelevant."
He said there should have been better protection to ensure there "wasn't that type of violence and assault".
"From the police's point of view... if they're present and they knew the event was happening, they should have been planning for that.
"Because someone was actually assaulted and it means that they have failed in making sure that event could have actually gone forward."
Mitchell said he hoped the police was doing "a serious review" of the operation.
"They need to be very clear with how they're going to deal with people's safety."
According to the Crimes Act, assault means the act of "intentionally applying or attempting to apply force to the person of another, directly or indirectly".
It also included the threat, by act or gesture, to apply that force to another person if the person making the threat has the ability to follow through or the appearance of being able to follow through.
Prime Minister Chris Hipkins said everyone had the right to feel safe in the community, including those who were disagreed with.
"The vast majority of people who were attending that protest over the weekend, did so in a way that was respectful, that upheld New Zealand's values as an inclusive society. I don't think we should judge that particular event based on the actions of a small group of people."
He said he would have been "proud to support" and attend the event on the side of the rainbow community, if he had been available.
National deputy leader Nicola Willis said she was "disappointed to see elements of violence" at the protest.
She said she didn't think anyone wanted "a situation in New Zealand where if a mob is unruly enough then someone else is prevented from their right to free speech".
"This is a moment for us to reflect on how do we respect each other and each other's views.
"There were elements of the protest that were peaceful, extremely well-meaning, but there were also elements that tipped it into something that was mob-like, and I think very regrettable."
Asked why she didn't attend the counter-protest, given she is a long-time Pride event attendee, Willis said she had other events on and had never heard of Posie Parker "before this whole hullabaloo".
"I don't think there's a need to put more attention onto issues that divide us. I continue to share a strong view that our trans people deserve to be affirmed in their identity."
Asked why she did not attend the trans right rally in Wellington, Willis said "I didn't think that the rally was serving a constructive purpose at that point".