Final pre-election polls either began recently, or will begin in the next day or two.
Pollsters all do things a bit differently, but my guess is most media polls are probably carrying out the majority of their interviews over the next three days, leaving the final couple of days to carry out call backs and appointments, and/or fill hard-to-reach quotas.
What does Kim Dotcom have to reveal on Monday?
Who knows…? If his bombshell has a major influence on voting decisions, the polls may pick some of this up in the last couple of days of fieldwork. The polls won't get that good a read on it though, because most interviews will be carried out prior to Dotcom's announcement.
A previous poll did reveal that more people believed Dotcom than the PM over when the PM first came to know about Dotcom, and that didn't seem to matter a great deal to National Party supporters at the time. For Dotcom's bombshell to influence National's support, it probably needs to be a WMD-level bombshell.
Why not wait until after Dotcom's announcement to begin fieldwork?
You could do this, but you'd have a very short fieldwork period, meaning limited call backs and a low response rate. You might pick up the influence of Kim Dotcom's announcement, but the results could very well be all over the place due to low response. In addition, you would have changed your methodology, so your results would not be comparable to your previous poll.
Personally, I wouldn't take this risk. But that's just me.