A Sydney shopkeeper charged with murdering three people by deliberately setting fire to his convenience store has been accused of giving bizarre and preposterous evidence.

Adeel Khan, 46, is alleged to have killed three people living above his Rozelle store, including a baby boy, by starting the blaze in September 2014.

In his closing address to the jury, Crown Prosecutor Mark Tedeschi QC said Khan's explanation for his purchase of 38 litres of petrol two nights before the fire does not make sense.

Khan told the court during his evidence that he wanted to have fuel in his car, so he could test the range of his vehicle after the low fuel warning light came on.

In addition to the fuel he pumped into containers at the back of his car, Khan also pumped petrol into the vehicle's fuel tank and never did the test.

"Why does he do it in the middle of the night?" Mr Tedeschi asked.

"It's so bizarre that you wouldn't believe it."

Adeel Khan also said he was planning to create a mixture of petrol, water and methylated spirits, to improve the performance of his car.

An expert told the jury such a mixture and the use of water in the fuel tank would only have caused the engine to splutter.

Bianka O'Brien, right, and her 12-month-old son Jude, both of whom were killed in the Rozelle blast. ( Facebook )

Other experts estimated that 20 to 30 litres of petrol was used to create the explosion and fire at Rozelle.

A jerry can containing 10 litres of petrol was found at the back of Khan's vehicle.

Khan claimed armed men held him up

Adeel Khan said during his evidence three armed men had help up his store for hours and only left a short time before the explosion.

Mr Tedeschi said Khan had left the keys in the ignition of his car, so he could escape as soon as possible after lighting the fire.

But Khan testified the men had earlier asked to be taken to his car and he started the engine.

"Oh, come on, really?" Mr Tedeschi said.

"No one's going to believe that. It's preposterous."

The jury was told the extent of preparation involved showed it was a very serious and organised plan to destroy the shop by fire.

"This was not some haphazard act of vandalism," Mr Tedeschi said.

He said the person responsible was "totally motivated" to destroy the building.