Powell said the RDA is currently in tough negotiations with DHBs about a new multi-employer collective agreement to take over from February 28.
"Our collective employment agreement has expired, we are currently in bargaining and our collective will cease to exist on the 28th of February next year," she said.
"At that point the only collective in existence ... if we haven't successfully completed negotiations will be the SToNZ contract.
"Inevitably the district health boards will start offering its [the SToNZ] collective to all resident medical officers [RMOs] and most won't be given a choice."
The union is entering further bargaining this week, talking about the potential of a new multi-employer collective agreement between DHBs and the RDA.
Powell told the Herald she thought DHBs were putting on a strong front in negotiations, retaliating against the previous disputes in 2016 and 2017.
If negotiations failed and the SToNZ contract was offered after their agreement's conclusion, Powell believes further strike action would be taken.
"The worry is that come February or March that DHBs will offer the SToNZ contract and no other," Powell said.
"If we don't get this settled and if the SToNZ contract is the one offered, it's almost inevitable strikes will take place.
"It's a really, really serious issue and we are worried about it."