'Beautiful and innocent baby boy' - photos of little William who was found dead with his dad in murder suicide

The distraught family of a baby boy killed by his own dad in a suspected murder-suicide has posted a tribute to the little boy.

Baby William, just six months old, was found dead in a car on a forest track in the Beerwah State Forest, on Queensland's Sunshine Coast, today.

His parents had split up and he was spending time with his father. But his dad failed to return him after the custody visit.

In a GoFundMe page created to help raise funds for William's burial costs, his mother's cousin wrote, 'It is with greatest sadness that I have to open up this go fund me page.

I am raising money for my cousin and her family after this horrific tragedy has occurred. This mother needs all the help we can give her so she can grieve for the loss of her child. She says she needs to go back to work to pay the mortgage and for her babies funeral.'

She added, 'Now this beautiful and innocent baby boy has been taken from her by the man who did nothing but make her life hell.'

Det Insp Dave Drinnen said the baby was reported missing on Monday after his father failed to return him to his mum following a routine custody visit. 

The father and the child failed to turn up as a result of some custody arrangements,' Det Insp Drinnen said.

'The mother was concerned... so [police did] welfare checks initially, before this was reported as a missing person investigation.

'This was treated as a high-risk missing person investigation ... to try and locate the missing person and child.'

But no amber alert was issued which would normally be the procedure, meaning that William's photo would be sent out to all media in a bid to find him. 

No weapons were found in the car and the cause of the deaths is yet to be established.

Police said the bodies had been in the car for 'some time'.