I'm applying for a visa before booking flights. The visa application requires a date of intent to travel. I'll be going mid-October but don't have a specific date yet as I haven't booked flights. If I was to put the 10th October, would it be a problem if I ended up travelling slightly after this date?
There is no obligation for you to arrive on your date of intent to travel. From United Kingdom Working Holiday Visa (click on "All visa conditions"):
You must come to New Zealand within 12 months of the date your visa is granted.
After you arrive in New Zealand, you can come and go from New Zealand as many times as you like before your visa expires.
It is completely flexible.
What matters is that once you are granted your visa you get 1 year to enter the country (unless different from Australia).