After trailing just 10-4 at the break on Saturday night, Samoa made 11 second-half errors, one of which led directly to a New Zealand try, to suffer their second largest loss on record against Kiwis.
"You can't toss garbage up and expect to compete, especially against a team that good," Parish said.
"Our second half performance, and look the Kiwis played very well, but we were our own worst enemies.
"We tried hard but we made schoolboy errors.
"[At half-time] I was pretty confident because I thought we played okay without playing great and were still in the game, but the second half was pretty disappointing."
The 30-point defeat was Samoa's heaviest loss in 13 games dating back to 2013, and post-match captain Frank Pritchard admitted it had served as a bitter blow to their progression as a squad.
"Definitely [it's step backwards], and it's a bit of a setback to lose our way," Pritchard said.
"We have got a lot to work on. The good thing about this is it's a tournament and this is only one game.
"It's hurting but we have got to quickly turn it around. We have got a tough team next week so we can't toss that up again."
Samoa have a week to recover before taking on Tonga in Hamilton, in what shapes as being one of the games of the tournament.
After watching his side complete just 20 of their 35 sets on their way to leaking seven tries, Parish said he had no doubt his men had plenty of improvement in them.
"The most disappointing fact to me is that we are a better team than we showed. I know we are better than that," Parish said.
"[We will] go back and regroup, we showed we can compete, we have just got to do it for longer periods.
"We aren't going to change too much."