The case is due to go to court in February.
Limited military, education and sporting sanctions were imposed on Fiji last year after the May coup led by George Speight toppled the elected Government of ethnic Indian prime minister Mahendra Chaudhry.
Fiji held elections last month, which observers declared were free and fair, prompting Australia to lift its sanctions.
But concerns linger that Fiji's new Government, formed by Prime Minister Laisenia Qarase, may not be constitutional.
Mr Qarase was obliged to invite Mr Chaudhry's Labour Party to join the majority SDL party in cabinet, which he did.
But Mr Qarase then rejected Labour because Mr Chaudhry had criticised SDL policies.
Mr Chaudhry has begun court action to have the Government declared unconstitutional.
Mr Goff said New Zealand welcomed the restoration of democracy in Fiji.
"It also accepts that Prime Minister Qarase, who has the confidence of the majority of Parliament, is properly Prime Minister of Fiji.
"However, the constitutionality of his cabinet remains in question.
"This was confirmed by Fiji's own Chief Justice, Sir Timoci Tuivaga."
The Commonwealth clearly believed an important constitutional issue remained unresolved. "New Zealand wishes to act collectively with the Commonwealth."
- NZPA
Full text plus audio:
Full text: