Paul Brian Edward Connelly, accused of trying to kill his sons in a car explosion using two gas cylinders, has pleaded not guilty to two counts of attempted murder.

His sister, Elizabeth Vanwatershoot, gave evidence at the Supreme Court in Burnie yesterday and said Mr Connelly visited her home hours before the car fire and said his wife told him she didn't want the boys.

"He said he loves them, but if she doesn't want them, I'll take them with me," Ms Vanwatershoot said.

Mr Connelly told Ms Vanwatershoot he had planned on December 4 to light the contents of two gas bottles and blow his house up after putting his boys to bed, the court heard.

He arrived at her house on December 5, crying, before they spoke, she said.

Less than two hours before the fire Mr Connelly's estranged wife told him via text message she wanted to have the boys for the night, it was heard.

She later received a message from his phone saying "I love you - I love you too", the court heard.

Firefighter Vaughan Thompson, appearing as a witness, said the car fire at Shorewell Park was the most intense he had seen.

Gas safety specialist Andrew Ayton, who investigated the gas bottles after the fire, said he had never known a fire caused by a gas cylinder in which its main valve was turned off, the court heard.

The trial continues today.