Commissioner defends Canberra's size

Royal commissioner Dyson Heydon has defended the nation's capital from accusations it's a small town.

During questioning at the royal commission into trade unions on Tuesday, the lawyer representing the Communications, Electrical and Plumbing Union, Brendan Docking, asked a plumber whether he'd heard of a union official causing trouble or shutting down sites elsewhere in Canberra.

"Given it is a small town ..." he started one of his questions, to which counsel assisting Jeremy Stoljar objected.

Mr Heydon agreed that Canberra was in fact a large city.

"It is a very important town and hundreds of thousands of people live here," the commissioner said.

While it may be a small town in hydraulic circles, it wasn't in reality, he said.

Mr Docking protested he was relying on earlier evidence to the commission, not his own opinion of Canberra.

"I've lived in it on and off for the better part of a decade or near it - I know what Canberra is, it is a large city," Mr Heydon admonished.

"Now back to the questions."