Mallard believed the House should sit because if serious issues arose, Parliament would need the ability to legislate.
"I think there's an accountability thing. Even if it's only five ministers or five opposition, you have the questions asked and answered. I think this is something which is quite important ... the public would expect an accountability mechanism still to be in place."
He's right, but there's an implication here that parliament might not sit. Which would be illegal. If a pandemic is declared, Parliament must sit as soon as possible. This is both for the accountability reasons the Speaker mentions, and because in such circumstances the government gets to temporarily change (non-constitutional) laws by fiat, subject to disallowance by the House (this is basicly what they did for Christchurch following the earthquake, only with better oversight). Which makes that scrutiny even more important - and it is reassuring to see that Parliament is planning to provide it if the worst happens.
For obvious reasons, they can't change the Electoral Act in this way. We have an election currently scheduled for September, so what happens if the pandemic threatens to stretch out that long? Parliament will have to legislate, assuming there are changes that can be made to make voting during plague easier and safer. Alternatively, the Prime Minister can delay the election until 21 November if required, just by changing her mind about the date - the announced date is just a promise, an intention, with no more force than that; 21 November is the last possible date we can legally have an election. But hopefully it won't come to that.