Simiona said he was "delighted" to be given spot kick duty and praised United for their "battling spirit".
"We could have given it to our top goal scorer Mitch but he happily gave it to me to finish off my hat-trick," he said.
"I think we underestimated United a bit. We knew they were going to come out firing and we started off a bit slow.
"But as the game progressed the boys really showed how they've been playing all season and it paid off in the end."
United drew first blood six minutes in when Watson pounced from close range after Ngongotaha failed to clear their lines, much to the dismay of the stunned home support.
But Ngongotaha quickly stepped up their midfield attack launching an assault on the United goal and would have drawn level sooner if it wasn't for a string of excellent saves from United keeper Andrew Gibbs.
Ngongotaha's Simiona had two powerful shots saved, one at point blank range, and Alan Ferguson and Miller also had chances expertly turned away.
But with seven minutes to go before half-time Ngongotaha's relentless pressure paid off when Simiona turned in a drilled Josh O'Sullivan cross after United should have cleared their area.
And with momentum very much on their side the home team cruelly went in with the half-time advantage after Simiona's long range shot deflected in off a stranded United defender minutes later.
Ngongotaha controlled the game after the restart thanks to their stylish pass and move play but United still looked dangerous up front with an attack led by Ash Gower-Rudman and Watson.
With the threat of dropping points in the air the Villagers introduced some much needed energy in midfield through substitute Sean Kim who made an immediate impact.
Kim was involved in the build up play that led to the ball falling to Miller's feet - the striker was then given too much time and space to unleash a superb long range strike that beat Gibbs in his right hand corner, after 78 minutes.
United's fate was sealed in the final minute of the game, with Simiona striking the ball low and hard to the left of Gibbs after a foul in the penalty box.
Despite the defeat United manager Ian Walker wasn't too disappointed with the result.
"We gave Ngongotaha a good run for their money," he said.
"We have been accused of 'parking the bus' in the past but we came out in an attacking frame of mind and I think the score line could have been closer.
"The team just needed to be a bit more clinical in front of goal because against top of the table sides you won't get a lot of opportunities."
Results:
WaiBOP Premiership:
Ngongotaha AFC 4 Rotorua United 1
SGS Papamoa Premiership 0 Old Blues Prem 2
Cambridge FC Prem 6 Katikati FC Ambria Premiership 1
Tauranga City Premiership 0 Otumoetai FC First Credit Union 3
Matamata Swifts Prem 3 Tauranga Boys' College 1st XI 0
Waikato Unicol AFC Prem 2 Melville United Prem 4
WaiBOP Championship:
Ngongotaha Champ 4 Hamilton Wanderers Champ 2
Cambridge FC Champ 1 Tokoroa AFC Champ 1
Matamata Swifts Reserve 2 Te Awamutu Edmond Judd Champ 3
Waikato Unicol AFC Reserves 2 Whakatane Town Champ 1
Taupo AFC Champ 5 Otumoetai FC Mens 1
West Hamilton Champ 4 Old Blues Champ 1
