Dr Reti was responding to calls for more security measures at some of New Zealand's smaller emergency departments at hospitals around the country.
The Association of Salaried Medical Specialists has claimed no security guards were on hand when the attack took place just before midnight on Saturday, which Te Whatu Ora/ Health New Zealand has denied.
It said security staff were on duty in the ED and were able to attend "within seconds".
A 62-year-old man at the scene was taken into custody and faces a charge of wounding with intent to cause grievous bodily harm.
In a statement, police said the victim and the offender were known to each other, and the victim received moderate injuries.
Reti told RNZ's Checkpoint programme today the alleged behaviour was "completely unacceptable anywhere in New Zealand - let alone in a hospital".
He has asked for an incident report which will be added to another report being compiled on security at all hospitals including EDs.
The government temporarily boosted the number of security officers at emergency departments over the summer.
Since then, eight 'hot-spot' EDs have kept the extra guards, with others receiving funding when they need it. That money is due to expire on July 1, and Reti refused to give an assurance of it being extended further because the information was "Budget sensitive".
He believed Wairarapa had received extra security staff until the end of February and defended the decision not to extend it.
"This is an isolated event; it's not a common occurrence at Wairarapa and those EDs where the security was reduced after the summer programme, it was done on the basis of what is the likelihood, what is the track record, what is the history of having assaults at EDs and Wairarapa wasn't one of those that came up."
Reti said he would wait for the incident report before making any further decisions on security at Wairarapa Hospital.
Keeping staff safe was a priority for the government, he said.
Aside from appointing security officers, other measures could be looked at such as a de-escalation (of violent incidents or threats) module which has been introduced at Auckland Public Hospital.
It could possibly be extended to other hospitals, he said.
At an earlier media briefing with Prime Minister Christopher Luxon on Tuesday, Reti said more work needed to be done to protect hospital staff and patients.
However, he said EDs were safe and people should go there when they needed to.
Luxon said it was important for staff to feel safe and the government understood that.
Dr Kate Allan, chairperson for the Australasian College for Emergency Medicine New Zealand, said beefing up security in place after the incident did not do anything.
"The incident has already happened. The staff and other patients need it and a timely manner when the incident is actually occurring."
EDs were vulnerable in a situation like the one that took place on Saturday night and could happen anywhere in the country, she said.
"We would like to see 24/7 in present security guards within every emergency department within New Zealand.
"They need to be integrated and trained as part of the workforce in the emergency department so they can be proactive rather than reactive when these things go badly."
As part of its 100-day plan, the government paid for 200 extra guards across some hospitals over summer, but that ended in February.
However, Health NZ was able to reallocate funding until July 1 this year, which meant increased security at eight "hotspot" ED's and surge security as needed - such as was now in place at Wairarapa Hospital.
