Citing the Supreme Court case of Attorney-General v Taylor, a declaration would not affect the validity of those acts, or anything done legally under those acts, but it would signal that the court considered the acts to infringe fundamental human rights in a way that could not be justified in a free and democratic society.
A declaration - or "constitutional prompt" as Otago University's Professor Andrew Geddis calls it - could be of assistance to Parliament if the issue were to arise in that forum. This was the case in respect of the Electoral (Registration of Sentenced Prisoners) Amendment Act 2020, which has since re-enfranchised people serving a prison sentence of less than three years, for example.
The Attorney-General opposed the making of the declaration saying it was not appropriate for the court to scrutinise the alleged inconsistency with the Bill of Rights Act, and suppose the court did so, the limit on the right was demonstrably justified.
What the court had to say:
Justice Doogue found that the declaration of inconsistency was a developing area of the law in New Zealand, saying the court should be slow to decline to hear claims relating to fundamental rights. However, he found the age limit to be demonstrably justified, and declined the application for a declaration of inconsistency.
"Age may be an imperfect proxy for maturity or competence; there will always be precocious children above, and incompetent adults below, the line wherever it is drawn. But a bright line is reasonable when establishing eligibility at the population."
A bit of history
Voting age dates back to the NZ Constitution Act 1852 where 21-year-old British men had the right to vote. The voting age was lowered to 20 years in 1960, then to 18 in 1974 as a result of student activism around the Vietnam War.
A 1986 Royal Commission review into the electoral system found all residents of whatever age are equally members of the community with rights and interests, which should be protected. It considered that as young people "are to be included as part of the community, the only justification for excluding them from voting must be lack of competence".
But: "by the age of 15 or 16, most young people have acquired a view of the social and political world that is not very different from the perceptions and understanding of mature adults".
In other words, those against changing the voting age suggest young people mightn't have the understanding to have a stake in the game, but you could say the same about middle-aged Joe Bloggs who votes for National because blue is "their colour", or because they have always done so. On that note, one has to question whether competency has ever been a prerequisite for voting?