Emergency services were alerted to the incident at 7.45am on Monday. The baby was found dead at the house and six other people were taken to hospital in varying states.
Gas inspectors were seen at the property the next day.
It's understood the dead baby's twin was in the most serious condition when rushed to Waikato Hospital.
However a Waikato District Health Board spokeswoman said all had now been discharged.
It's believed the twin was the last to be discharged yesterday.
It's unclear how long the panel heater had been leaking. The heater was not a portable or patio gas heater, which need ventilation and have been the cause of carbon monoxide poisoning deaths in the past.
Wall gas panel heaters are usually flued and vented.
A funeral for the little boy will be held in Hamilton today. His death has been turned over to the coroner but a police spokesman said enquiries were ongoing.
The coroner's office said it could not comment.
The Plumbing, Gasfitters and Drainlayers Board said it could not comment on the incident because it was currently under investigation by Energy Safety.
Energy Safety is a division of WorkSafe. A WorkSafe spokeswoman confirmed its Energy Safety team was now investigating the incident.
"As the matter is now under investigation I am unable to make any further comment," the spokeswoman said.
In July last year six members of a family were hospitalised after a massive gas explosion at their home rocked the suburb of Northwood.
A contractor had looked at gas issues at the house a day earlier.
