Two family members are assisting police after the suspicious death of a 40-year-old man in Mount Isa in north-west Queensland.

Authorities said the man was found with critical injuries on the front porch of a house in the Mount Isa suburb of Happy Valley about 4:00am.

The man, who could not be revived, died in the low-set house in Emerald Street.

Police said the cause of death was yet to be determined but they were treating it as suspicious.

Mount Isa District Detective Inspector Anne Vogler said it was unknown what injuries had been sustained.

"Obviously that is what we will be looking to obtain from an autopsy," she said.

Detective Inspector Vogler said the two people in custody were known to the man.

"All parties at the house are known to each other — they are related through family," she said.

"We don't believe anyone else was involved, or even at the house."

A crime scene has been declared but charges are yet to be laid.

Detective Inspector Vogler said police were still at the scene.

"We're still completing our searches, so once that's finalised we will have a better understanding of what exactly did occur," she said.

Neighbour Sarah Jones said she was woken by a siren at 4:00am, but otherwise heard nothing.

Ms Jones said she found the news quite scary.

Police said they were still trying to contact the man's next of kin.