Freedom of speech includes freedom not to speak.
If you want to force someone to answer a question you would need to drag them into a court proceeding or Royal Commission or similar. There would be law changes required if ministers are currently exempt from those issues. I'd question the point of going to all that trouble. If someone has a mind to be uncooperative, their answers can be just as frustrating and unhelpful as their silence.
The Coleman-Fergusson interview underlines an apparent issue with current NZ journalists. They only prepare one line of questioning. When that stalls all they have the wit to do is repeat, repeat, repeat.