With the World Cup due to kick off this month, the Rugby League International Federation (RLIF) have ruled that Parramatta prop Fuifui Moimoi and Wests Tigers winger Taniela Tuiaki are both ineligible to represent Tonga.
The RLIF pointed to the players switching allegiances to play for New Zealand last year.
Earlier this week, Tupou was named in the Tonga squad after originally being overlooked by Australia. But an injury to Michael Crocker resulted in him being recalled into the Kangaroos squad on Thursday.
"I think it's a big joke," Dymock told Foxsports.
"I just don't understand, after the World Cup everything all starts again. So why can't they just do it now?
"I just want that explained to me. What's the difference either now or after the World Cup?
"People want to see these players. If they're not playing for New Zealand or for Australia they're trying to help out the emerging nations.
"That's what they're trying to look at, internationals to be back where they used to be, strong internationals instead of these one-sided games."
RLIF rules allow one application to change to another nation in any two years. Moimoi and Tuiaki both switched from Tonga to the New Zealand for the 2007 tour of England and France.
Players can also represent a different country after the upcoming World Cup.
Dymock also said in his interview with Foxsports that he understood that New Zealand were accepting of the selections.
"I spoke to (New Zealand coach) Stephen Kearney and I'm pretty sure he would be fine with it,
"I don't mind the Tupou one, I can cop that on the chin."
In what is shaping up to be a strong Tonga squad, two other players, Cowboys-bound prop Antonio Kaufusi and Warrington's Louis Anderson were cleared to play by the RLIF. Fiji's Parramatta Eels and Kangaroos centre Jarryd Hayne, and Samoa's Bulldogs halfback Ben Roberts were also given approval.