-With AAP.

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A man gassed his entire family to death after his wife discovered he was having an affair with a 17-year-old girl in the Philippines and demanded a divorce, an inquest has heard.

Maria Lutz, 43, Fernando Manrique, 44, and their children Elisa, 11, and Martin, 10, along with the family dog Tequila were found dead in their Sydney home in October 2016.

An inquest today heard the Sydney father was in debt, and there was “little cause for doubt” that Mr Manrique was responsible for the family’s death.

At the opening of the inquest into their deaths at Lidcombe Coroners Court, counsel assisting Adam Casselden said Mr Manrique had planned the murder of his children and wife over some time.

"Maria had no awareness of, or involvement in, Fernando's plans."

The court heard Ms Lutz was a "dedicated, loving mother" to her two severely autistic children.

She was excited about the possibility of going back to work after finding out they would be receiving $50,000 from the National Disability Insurance Scheme, and was set on leaving her cheating husband.

"She was looking forward to life without Fernando," Detective Sergeant Timothy Pooley told the inquest.

Mr Manrique, meanwhile, was in "dire straits" financially, Det Sgt Pooley said.

He owed at least $15,000 to the tax office, had $28,000 in credit card debts and was struggling to pay off the family's two mortgages totalling more than $500,000.

The family trust account had just $6 in it.

Despite his debts, Mr Manrique was supporting a woman he was in a "serious relationship" with in the Philippines, where he travelled for work each month.

The woman, known only as Jamilyn, was 17 when she met Mr Manrique in 2015 at a bar where she worked. He had convinced her to quit her job, promising he would buy her a house.

But when his wife discovered the affair, Mr Manrique set about planning how to kill himself, along with his family.

The inquest heard he ordered two cylinders of carbon monoxide 10 days before he murdered his family.

He arranged for them to be delivered to his friend Jairo Campos' house, who was led to believe he would be using them to run tests on "gas released by cars in underground car parks".

Mr Manrique paid him $400 for storing the cylinders.

Deputy state coroner Elaine Truscott made a brief statement midway through proceedings to thank Ms Lutz's friends for "honouring her" by attending court and providing statements to police.

"[It is] very clear Maria loved her life, loved her children and had every intention of continuing a very loving, giving and productive life with her children," she said.

Mr Casselden said it was a "real concern" how easy it was for Mr Manrique to purchase the gas he used to kill his family.

The inquest continues.

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