On the fifth day of an Alexandra man's trial on arson and intentional damage charges, Catherine Trevathan told a jury in the Dunedin District Court the tyre and other materials had been deliberately set alight after they were placed under the corner of a relocatable building in the yard.
All four buildings on the site were involved in the fire.
But the Portacom office building where the fire had originated sustained most of the damage,
Ms Trevathan said.
Electrical appliances, cables and multi-boxes in the remains of that building were examined and discounted as the source of the fire.
Ms Trevathan, who will continue her evidence on Monday, is the 40th of the Crown's 48 witnesses in the trial of Aaron Gourlay (43), a former Wastebusters employee.
Gourlay, represented by Adriana Pinnock and Deborah Henderson, denies responsibility for the blaze, which damaged several buildings in the Wastebusters yard about 8pm on September 3, 2015.
He also denies setting fire to cars belonging to two former work colleagues and his former partner and puncturing the tyres of two other vehicles, all in 2015.
His trial is expected to be completed next week.
Judge Kevin Phillips and the jury had heard evidence of Gourlay's volatile relationship with a former manager of the Wastebusters centre, although it was acknowledged other employees also had difficulties with the same manager.
There has been evidence the defendant spoke to various people about setting fire to one of the cars belonging to a former colleague, although the defendant later claimed a "bomb" said to have been used to start that fire had been made to comfort his partner's son, who was afraid characters in a television programme The Living Dead would "get him".
A former colleague whose partner became involved with Gourlay a few months before the Wastebusters fire agreed he had been angry with the defendant for causing the end of his relationship.
But he had "moved on" from that and had not blamed the defendant for causing the Wastebusters fire, the man said.
The woman involved said Gourlay was an alcoholic and he lit a fire in her car in December 2015 after she told him to leave.
Senior Constable Rowan Williams told the court he started an arson investigation after speaking to Ms Trevathan, who said there was evidence the fire had been deliberately lit.
While collecting debris from the area, he noticed a distinct smell "like paint thinner".
He took photographs of an area where a tyre was found and could see clear patterns of unidentified liquid having flowed across the bitumen away from the tyre.
