A Sydney engineer who killed his pregnant wife and eldest son in a double-murder suicide car crash planted bombs in the vehicle that failed to detonate.

Darren Milne, 42, and his wife Susanna died instantly when he deliberately slammed his Toyota Corolla at high speed into a tree on Enterprise Drive on the NSW Central Coast, the Central Coast Gosford Express Advocate reports.

His 11-year-old son Liam was treated by paramedics but died at the scene.

Liam's brother Benjamin, who was 7 at the time of the crash on January 30 last year, was critically inured but emerged as the sole survivor.

An inquest into the tragedy has heard that Mr Milne had planted in the bonnet of the car two petrol bombs, which failed to detonate.

Leading Senior Constable Peter Mason, of the Forensic Services Group that took the ­engine bay apart, said investigators discovered a "sophisticated" circuit of wiring and plugs connecting three 12-volt batteries to two metal flasks, each containing about 500mL of fuel.

One was attached to the engine bay behind the right headlight while the other was believed to have been placed behind the left headlight. However, due to the force of the crash, the second flask was found in the back seat.

The inquest at Wyong Local Court also heard that Mr Milne had been diagnosed with "some kind of depression or ADD" in 2007 and had intermittently taken medication since then.

The inquest also revealed that the Ausgrid engineer had meticulously planned the murder-suicide, made a check-list of things to do, and conducted up to 10 practice runs before finally wrapping the family car around a tree.

Detective Senior Constable Karen Kennedy said an iPad with what was effectively a suicide note dated September 20 was found in the wreckage .

"From this point on I need to be totally focused, forget everything else," the note read.

"Take DVR (dash camera) out of the car so as to not raise suspicion. Copy work personal stuff to portable disk. Start taking personal stuff home.

Snr Cst Kennedy said the main motivation for the tragedy appeared to be his son's diagnosis with Fragile X syndrome, a genetic condition that causes intellectual disability, behavioural and learning challenges and various physical characteristics.

As well as the effect it would have on his children, Mr Milne believed that he and his wife would not be able to cope with the challenges their disabilities would present.

"It's not worth it, neither of us have the skills to make it work. We have both given it our best shot over a long period of time. There is too much conspiring against us," the note said.

"L & B are both happy, B doesn't yet know, it is a good time to go. It is only going to get tougher as time goes on... They are going to have to manage ADD + Diabetes, it is going to be too much. They need to exercise & manage their health, it is going to be hard to see this fail.

"Things are going to get progressively harder for Ben. He hasn't seen any malice or bullying yet, but it is coming."

The inquest has finished hearing evidence and the magistrate will hand down his findings in July.

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Liam (left) and Benjamin Milne.