The Rugby Football League have imposed the fine - which will be suspended until the end of the season - after a restart had to be halted as Powell made his way from one side of the field to the other shortly before half-time.
A statement read: “Powell’s punishment, for breaching Operational Rule D1:13, takes into account his acceptance of the breach, and assurance that it will not occur again – as well as the club’s approach and assistance with all aspects of the matter.
“The £5,000 fine has therefore been suspended until the end of the 2019 season, and will only become payable in the event of further breaches of the Operational Rules relating to non-field personnel entering the field of play outside of RFL Regulations.”
Castleford had just fallen 18-0 behind against Saints - they went on to lose 42-12 - and in the post-match press conference the coach explained: “I was just having a quick word with (winger) Greg (Eden).
“But I just wanted to get to the other side and try and organise things for half-time.
“I stopped the game, though, didn’t I so I didn’t quite make it!
“It’s just disappointing; your team’s getting belted and there’s no way down and around from there (gantry to dugout) so it’s hard work.
“We need a zipwire to get down and across the field.
“It’s a disappointing night. I won’t do that again.”