The big win has rekindled three-time defending champions Thailand's hopes of grabbing one of the two semi-finals spots from the group, which also include Vietnam, Indonesia, Singapore and Laos.
It was the War Elephants' first win in the biennial tournament after they suffered a shock 2-0 defeat to Indonesia in their first game on Tuesday.
The side, which saw several changes, delivered a much-needed morale-boosting victory in Manila.
Nishino made six changes to the squad that started against Indonesia, bringing in Korraphat Nareechan, Chatchai Saengdao, Rattanakorn Maikami, Sittichok Paso, Jaturapat Satthum and Wisarut Imura and the side delivered a much-needed morale-boosting victory in Manila.
A Thai victory over Brunei, who were routed 6-0 by Vietnam in the first game on Monday, was always on the cards and they came good on their pre-match promise of producing a goal-fest.
Nishino admitted that Thailand must win their three remaining matches, including one against arch-rivals and co-title favourites Vietnam, to assure themselves of a place in the semi-finals.
"After we lost 2-0 to Indonesia in the opening game, I told my players that the most important thing was to be more aggressive and show more hunger," said Nishino.
"We had to make changes in the starting line-up and after the break in both games because we have not been getting the rhythm we wanted.
"We only have a limited number of players and we need to rotate our players well in the remaining three matches.
"We have no other choice left but to win all three matches to secure a place in the semi-finals and to achieve that, we depend on the fitness of our key players."
Brunei coach Aminuddin bin Jumar said: "I cannot compare my team with the two teams (Thailand and Vietnam).
"I can only say that both Thailand and Vietnam team are quality teams and they play stylish football."
The Thais did not find the going easy in the first half as it took them 43 minutes to get their first goal when Supachai Chaided nodded home a Chatchai Saengdao header.
The second followed quickly with Saringkarn Promsupa setting up Jaroensak Wongkorn.
The introduction of Supachok Sarachart and Suphanat Mueanta in the second half improved the Thais. They scored their third goal in the 52nd minute when skipper Anon Amornlertsak curled in a free kick.
Thailand scored their fourth 20 minutes later thanks to a Sarayut Sompim header and in the final 20 minutes teenager Suphanat Supachok and Anon wrapped up the 7-0 rout.
Thailand will play their third group match against Singapore on Sunday.
Also on Thursday, Indonesia posted their second straight win when they beat Singapore 2-0 to share the top spot with Vietnam, who routed Laos 6-1, on six points, three more than Thailand.