The first try of the match came at the 17-minute mark when Lance McDonald latched on to a Charles Piutau pass on Auckland's 30m mark for McCashin to convert.
At the 25th-minute mark both sides looked to find space through helter-skelter rugby, kicking behind their opponents' defensive line, but either the final pass or a knock on at the ruck saw attacking opportunities fade away.
But McDonald scored his second in the 28th minute, when the Steamers worked a move off the back of a 30m attacking scrum which went through several hands before the fullback went over on the left hand side, taking the Steamers out to a 18-6 lead.
Bay of Plenty had the chance at the 34th minute to score their third try when Luke Braid, in his 50th match, turned over Auckland's attacking ball on the 22. The ball was kicked downfield for a chasing Lelia Masaga who held up Piutau. The Aucklander was penalised for not releasing which McCashin nailed to take the score to 21-6.
McCashin was able to increase the score to 24-6 when Auckland were penalised for a late tackle right on halftime.
After the break, Auckland looked to use their bigger players to run at the Steamers defence. The home side were eventually penalised for not releasing the tackler and Anscombe added three more points to make it 24-9.
Auckland were in three minutes later for their first try. After sustained pressure, Anscombe got a mismatch against Braid, running around the No8 to run 22m to score and convert.
Despite the manic start from the visitors, the Steamers sparked an attack at the 60th minute thanks to a Masaga run down the right flank which showed Auckland were susceptible to a high-paced game. Although the attack didn't result in a try, the pressure resulted in a McCashin penalty to move the score to 27-16.
Phil Burleigh scored the Steamers' third try three minutes later after he snaffled a perfect box kick by halfback Josh Hall on the 40m line to race all the way to the left-hand corner. McCashin's kick was just to the left of the posts but the lead was now 16.
The game was sealed with two minutes to go.
The Steamers, hard on attack on the right side, spun the ball wide. The resulting ruck 5m out saw McCashin sneak down the blindside to score in the corner for a bonus point.
Last night's results
Bay of Plenty 37 (L MacDonald 2, P Burleigh, N McCashin tries; McCashin 5 pen, con), Auckland 16 (G Anscombe try, 3 pen, con). Halftime: 24-6.
Counties Manukau 43 (A Cathcart, D Bason, S Stowers 2, F Halai tries; B Kerr 4 pen, 3 con) Otago 12 (H Parker 4 pens). Halftime: 13-12.