The NZ Herald writes about the possibility of an eighth NZ First MPs if the electoral petition is successful, and if Peters wins any subsequent by-election.
They also talk about the option of him not resigning as a List MP, so the proportionality is not upset.
I am unsure that you need an explicit resignation. If a writ is returned declaring him as the MP for Tauranga, it may well be that he automatically vacates his place as a List MP.
I haven’t checked, but I seriously doubt you can hold two writs at the same time. By the same principle if you were MP for Auckland and then contested a by-election in Dunedin and became MP for Dunedin, your earlier seat would be vacated I believe.