Consent should be opt-in.
The title says it all. Data should only be shared with positive permission from the subject of that data. If the government wants to share it has the responsibility to ensure that every participant is fully informed, which can only be the case if explicit permission has been given.
The likelihood that the number of people who would give permission would probably be so small as to make the whole excercise worthless says something about the trust people have in the apes in parliament.
Biting the hand that feeds IT
