New Zealand is at Alert Level 4 lockdown following an outbreak this week of the the highly infections Delta variant of Covid.
New rules make masks mandatory for supermarkets, petrol stations, dairies and pharmacies which are open.
The Foodstuffs chain, including Pak'nSave and New World, will turn people away if they're not wearing a mask.
At Countdown, masks will be provided to shoppers who left theirs at home, but would not automatically refuse entry.
Countdown spokesperson Kiri Hannifin says the incident happened on Friday afternoon at the Church Corner store.
She said the man, who has been previously trespassed for abusive behaviour, was refused entry to the store.
He then spat on the security guard and refused to leave, prompting a police callout.
Hannifin said the guard is being tested for Covid-19 and receiving support.
"It is always incredibly disappointing when our team are subject to abusive behaviour, but it is particularly distressing when this involves spitting at a time when the country is experiencing a community outbreak of Covid-19."
Hannifin said earlier this week that Countdown has a no mask, no entry policy and will make them available for shoppers and encourage them to wear them.
However, she said they have been told by police that if customers refuse to wear them, then they will have to allow them to enter their store.
"When we did try to manage things like [QR code] scanning when you came in ... we got a bit of conflict.
"So what we're going to do is encourage our customers, but if they refuse or if they become aggressive or violent or angry, we're just going to walk away from that conflict.
"We're not going to engage. I don't want my team to get hurt."
Hannifin said anybody acting aggressively towards their workers would have the police called on them.
