A Sydney family, including two children, were killed at the tragic conclusion of an "evil plan" that was perpetrated by their father over several weeks, an inquest has heard.

The dad also gave a "parting gift" of more than $3000 to a teenage lover, despite the family being almost broke.

The bodies of Fernando Manrique, 44, his wife Maria Lutz, 43, and their children Elisa, 12, and Martin, 11, were found in the family home in Davidson, on the city's upper north shore, on October 17, 2016.

All had died of carbon monoxide poisoning. Manrique is suspected of rigging up an elaborate mechanism to spread lethal gas from two gas cylinders into the bedrooms of his wife and children in an apparent murder suicide.

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The painstaking steps Manrique took in the weeks leading up to the slaying of his family were detailed before an inquest into the four deaths, which concluded today. The inquest examined how Manrique was able to relatively easily procure get a highly toxic gas and get it delivered to a residential address.

Counsel assisting the coroner Adam Casselden told inquest today that Maria, Elisa and Martin likely died in their sleep given they were found in their beds dressed in pyjamas.

When the bodies were discovered in 2016, questions were raised about whether Maria had any involvement in the deaths. Mr Casselden was at pains to put an end to that speculation.

"It is inescapable that Fernando planned the deaths of his family over at least 16 days and was solely responsible for constructing a system to pump carbon monoxide into the family home and that Maria had no information of the plan," he said.

Maria Lutz, far left, her son Martin, husband Fernando and daughter Elisa. Photo / News Corp Australia
Maria Lutz, far left, her son Martin, husband Fernando and daughter Elisa. Photo / News Corp Australia

Manrique had moved out of the family home after his marriage broke down. He also had a teenage lover in the Philippines, a country he visited frequently on business.

The inquest heard Maria allowed her husband back into the family home in the weeks leading up to their deaths to enable him, she told friends, so he had somewhere to bed down while he found somewhere else to live.

It's thought that during that fortnight, while the kids were at school and Maria was out of the house, Manrique painstakingly built the system to distribute the gas.

"There is compelling evidence Fernando had taken steps to conceal it from his wife such as locking the gas cylinders in the shed, out of sight," Mr Casselden said in his closing statements.

"Why would he need to take those elaborate steps if Maria was in some way a knowing participant in his evil plan?"

Mr Casselden said it was "open to interpretation" if Manrique meant to kill himself as well.

Unlike his wife and kids, he was not found in bed but rather sprawled face down on the hallway floor. A bag was packed with mens clothing and $3400 had been sent his lover — a 17-year-old in the Philippines by the name of Jamielyn who he had been seeing on his business trips.

The cash transfer took place even though Manrique owed thousands on credit cards and to the Australian Taxation Office and had barely any money in the family's bank accounts.

But, said Mr Casselden, knowing carbon monoxide was so poisonous it was perplexing why a man set to flee had locked all the doors. The packed suitcase could easily be that of any frequent traveller and, besides, he had no flights booked.

Fernando Manrique (right) with his son Martin. He is accused for killing his whole family. Photo / News Corp Australia
Fernando Manrique (right) with his son Martin. He is accused for killing his whole family. Photo / News Corp Australia

Mr Casselden also pondered if the thousands sent abroad could be a "parting gift" to Jamielyn.

He said he was "unable to make a final decision" on the reasons behind Manrique's death.

However Douglas Spencer, who appeared for the Manrique family and estate, said the "weight of evidence supports the conclusion that he intended to die with his family" that night.

"If someone was going to murder his family and get away, there would be packed bags in the car to make a quick escape. But there was no evidence of that kind," Mr Spencer said.

He said it was "inexplicable" that Manrique would not have protective equipment to protect himself from the gas.

"If he intended to do this terrible thing and escape you would think he would have had travel plans because he would have known there would be a manhunt like no other, and that would have come very quickly," he said.

Mr Spencer said the Manrique family had "lost a sister-in-law and a brother who brought about this tragedy in circumstances that are unimaginable and they have to come to terms with that".

On Wednesday, Maria's close friend Peta Rostirola said there mum was "brave, beautiful and brilliant".

"She knew Elisa and Martin loved her and that made her love them harder and try harder. Life loving Elisa and Martin was all she needed," Ms Rostirola said.

The coroner, Elaine Truscott, will deliver her findings on 17 May.

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