Can I challenge rent raise based on landlord's reasoning?

This is the second year in a row my landlords (through a letting agent) have increased my rent.

I just got an email from my property manager stating my rent is 'below the current market rates'.

Last year it was a $10 per week increase, this year it's another $20 per week.

It doesn't sound like much, but like many people right now, I'm slowly sinking financially and it's a big deal amount for me.

I have seen the news reports about rents dropping in most places around the country. I live in Auckland, which is one of the areas having seen a drop in rents.

I have emailed my property manager pointing this out and have asked to see proof my home is currently renting 'below market' rates.

What are my options around contesting the latest rent increase?