The father of fatal balloon pilot Lance Hopping suggests passive smoking may be to blame for the cannabis found in his blood.

An interim report released by the Transport Accident Investigation Commission said ESR tests showed traces of the drug were found in his bloodstream and urine.

On January 7, his balloon struck powerlines near Carterton, caught fire and plummeted to the ground, killing Hopping and his 10 passengers.

The findings were a surprise to Hopping's elderly father Robert.

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He told the Herald on Sunday on Friday he had never seen, or heard about, his son smoking cannabis.

"It's very strange to me."

He questioned whether passive smoking of the drug - sharing an enclosed space with people smoking the drug - could return a positive test.

However, Canterbury Health Laboratories toxicologist Grant Moore said he would be surprised if passive smoking would result in cannabis being found in blood or urine.