Both teams were forced from the ground on several occasions as the 1000-strong crowd dashed for cover.
Having already been off twice, the Knights reached 82-4 in 11.3 overs when the rain got heavier, with the Firebirds set a revised target under Duckworth Lewis of 90 runs off 11 overs to win.
Despite losing opener Michael Papps for six in the third over and Zimbabwean Brendon Taylor in the fifth over for three, Wellington did it easy with Michael Pollard and James Franklin combining for a matchwinning third wicket partnership of 58 off 36 balls.
Despondent Northern Districts chief executive David Cooper said the financial hit would be significant as crowds stayed away during the most lucrative time of the year for the six major associations.
"We can't buy a trick with the weather right now unfortunately [although the same could be said of the team], and when you come across at this time of the year to the beach, and like all New Zealanders would like it to be sunny, it's hugely disappointing when there's nothing there."
The Bay Oval last summer hosted 10,000 fans for two Twenty20 games with a scheduled third game washed out - twice as many per game as any other HRV Cup venue - so Cooper had every reason to be confident the tills would be ringing.
It was too early to say what financial toll the rain-affected games would have but it would be significant.
Northern could have expected in excess of 5000 punters through the gates on New Year's Eve but instead got none, while yesterday should have yielded another crowd of around 3000.
"We plan for good weather at this time of the year but factor into our planning one wet day. So far we've had two, but both times we've got games in that has had a massive impact on what we draw, with the bottom line a flow on from that," Cooper said.
"New Year's Eve should have been a big day but instead we got no one, and we would have expected to have backed up today with the Murali [spinner Muttiah Muralitharan in his first and last visit to Mount Maunganui] factor.
"We budget incredibly conservatively anyway but each year it's becoming difficult to get that right.
"Two years in a row we've taken a big hit but it's not the end of the world, although with two more games here [today against the Central Stags and on Sunday against Auckland] and we've got our fingers crossed that the weather plays its part."
Fingers must all be crossed that the Knights can remedy their woeful HRV Cup form, and fast, after their short-version form was again found wanting in a stop-start day.
Pollard was the mainstay for Wellington with a 31-ball 43 while Franklin's 27 included two sixes; the second winning the game as he put Knights skipper Scott Styris over the square leg boundary.
Earlier, the Knights were again stymied by a lack of momentum with the bat and the persistent rain delays, with Daniel Vettori's 26 at the top of the innings and Styris' unbeaten 20 their best as Muralitharan, Mark Gillespie, Andy McKay and Luke Woodcock grabbed regular wickets for the Firebirds.