Jackson was suspended by FootballSouth after its disciplinary committee found he assaulted referee Bernie McCone after a match against Green Island at Tahuna Park on Easter Monday.
Caversham accepted Jackson made contact with the referee with his shoulder but felt the incident warranted a misconduct rather than an assault charge.
The club appealed but NZF ruled ''there was no, or insufficient, evidence in the letter of appeal ... to overturn the factual finding of assault''.
Caversham spokesman Steve Fleming said the club was ''bitterly disappointed'' with New Zealand Football's decision.
''We don't think that common sense has prevailed,'' Fleming said.
''The way the rules are set up, the way the referee sees it is seen as 100% correct.
''It comes down to how the rules are interpreted. We were hoping for a misconduct towards the referee; instead, they've got him on an assault charge.
''We put our case to New Zealand Football but they were quite happy we hadn't proven anything.''
Caversham received New Zealand Football's decision on Tuesday night and is still considering whether it will pursue an appeal through Fifa.
''We've got a meeting ... and have to go through how much time and energy we throw at this. We want to make sure we are doing the right thing by the boy. He has been a valued member of our club for many years and we don't want to give up on him.''
FootballSouth general manager Bill Chisholm said he was pleased the decision had been upheld by the national body and felt the issue had been dealt with fairly.
''If you determined the referee was assaulted then six months is the minimum,'' Chisholm said.
''The key thing was to establish whether it was assault first, and that has been upheld. There was intentional physical contact with the referee and that constitutes assault. While we all agree it was at the lower end of the scale, it is still assault.
''We can't have officials being man-handled or assaulted. That is the beginning and the end of it.''
Jackson is a talented player who has represented Otago and also attended Marshall University in the United States for four years on a scholarship. Last year, he had a trial with the Wellington Phoenix.
