You are entitled by SNZ rules to enter on the day.
Sorry Buddy, I am gonna have to disagree with you here and state that the above is just another "Speedway Urban Myth."
A promotion running any NZ event MUST accept all entries up to the prescribed "Closing Date" as per their entry form. (This is one of the reasons why ALL NZ event entry forms have to be approved by SNZ.) After the closing date it is entirely up to the Promotion as to whether they would accept "late entries", especially those that decide to rock up on the day and expect to compete.
If you, as a competitor, are racing the clock (and/or bank balance, wife, mistress, etc) you still need to get your entry in, before the closing date. If you don't get there, then all you stand to lose is the $20 - $25 entry fee. A very small price to pay for guaranteed entry, especially if you finish the car the morning of the event.