I believe it's the nature of a Royal Commission compared to a usual Court.
The common law (and criminal law) protection against self-incrimination does not apply to those called to give evidence to a Royal Commission. The so-called “coercive powers” of a Royal Commission enables a Commission to compel people to attend and answer questions under oath. Failure to comply with a summons issued by a Royal Commission may result in a fine or in some circumstances, imprisonment.
The same sort of penalties apply if requested documents are withheld.
Here is more information: http://commissionwatch.com.au/what-is-a-royal-commission/
Pants.