Lomu was on the bench for the Cardiff club Old Illtydians in a Swalec Cup second-round tie against Cwmllynfell on Saturday when players started exchanging blows.
"There was a bit of a fracas going on between the forwards and John ran into the middle of it," Wyn Griffiths, the Cwmllynfell fixture secretary, said.
"I honestly think his intention was to separate the players involved but as he arrived in the thick of it he got a bit of a clout. So he gave one back and as a result he got sent off."
Cwmllynfell won the match 22-19, but Lomu's hopes of a cup run rest on the outcome of a disciplinary hearing that could imposed a ban ranging from four to six weeks.
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