The contest displayed the hallmarks of a opening round match as both sides were unable to assert any authority with handling errors and indiscretions ensuring plenty of whistle from Australian referee Paul Marks.
The Pacific Islanders were heavily penalised throughout and spent 20 minutes of the second half a man down through sinbinning after two unnecessarily dangerous tackles -- illegalities that contributed to their defeat.
Fullback Sione Tu'ipulotu was binned in the 48th minute after Maori blindside flanker Liam Messam was shoulder charged as he in-passed to the first tryscorer Tamati Ellison -- and seven minutes after his return second five-eighth Epeli Taione was yellow carded for a high shot on impressive No 8 Thomas Waldrom.
Though the Maori side only added three points while Tu'ipulotu was sidelined, the numerical advantage eventually told second time round when discarded All Blacks halfback Piri Weepu loomed in support to cross untouched after a strong burst by wing Hosea Gear down the left hand touchline.
Weepu's score in the 72nd minute sealed the outcome though the Tongans rarely threatened and had to settle for three penalties by pivot Fangatapu Apikotoa to stay vaguely in touch.
Although the Tongans occasionally enthused a vocal support base by bludgeoning valuable metres, they rarely ventured within range of the Maori goaline.
The scores were locked at 3-3 at the end of a turgid opening half highlighted by the whistle of Marks and the trend continued after the interval.
Callum Bruce missed his first two handy shots at goal but atoned with success from four of his last five to give the Maori breathing space.
In the other match today, Fiji beat Samoa 34-17 in Lautoka, Fiji where the Maori side will be in action on Saturday. Tonga play Japan in Sendai next Sunday.
New Zealand Maori coach Donny Stevenson agreed it wasn't an attractive game at times but he was relieved to bank the win.
"We looked like a team that had been together for less than a week to be honest so we're happy to take the few positives out of that and move on," he said.
"We never got any momentum going, we didn't really string phases together. It was the first week and hopefully we'll improve on that."
Although it took Weepu's try to guarantee success, Stevenson felt his side always had the game in hand.
Tongan captain Nili Latu, who played the entire match despite suffering from the 'flu, said moments of ill discipline obviously undermined his side.
"We thought we had a fair chance of winning the game if our defence and discipline were good. I guess our discipline didn't work that well, it cost us the game."
New Zealand Maori 20 (Tamati Ellison, Piri Weepu tries; Callum Bruce 2 pen 2 con) Tonga 9 (Fangatapu Apikotoa 3 pen). Halftime: 3-3.
