The shooting happened at a rural property near Margaret River.
Homicide detectives were investigating but police were not looking for a suspect - suggesting a murder-suicide although this was not confirmed.
Authorities would not say how the victims died, but confirmed that two firearms were found at the scene.
The commissioner would not speculate on a motive but said that, at this point, police had no concerns about wider public safety.
Dawson described the incident as a "tragedy" and "horrific incident".
"Regrettably, these tragedies do take their toll", Mr Dawson said.
Upon arrival, 4 children and 3 adults were found deceased.
Osmington is a 10-minute drive east of the Margaret River township.
Australia's gun laws are widely acclaimed as a success, with supporters including former President Barack Obama saying Australia has not had a single mass shooting since they were implemented. Four children and three adults'.
"It's sending shockwaves through the whole community - we're all linked in one way or another, every family", she told WA Today.
Mr Dawson said: "This will be a very large scale and detailed investigation".
He said the dead appear to be residents of the property, but next of kin were still being communicated with and so no more details were immediately provided.