Some stats I've picked up over time from NZ Soccer Annuals and newspapers
showing Average crowds for National League
1971 (2nd year, 10 teams - Listed in finishing position)
1 Eastern Suburbs 1274
2 Mt Wellington 1769
3 Christchurch United 1283
4 Blockhouse Bay 1274
5 Gisborne City 679
6 Dunedin Suburbs unknown
7 Mt Albert-Ponsonby 845
8 Stop Out 662
9 Wellington City 822
10 Western Suburbs 946
by 1980 they had 12 teams but attendances had dropped
1 Mt Wellington 1147
2 Gisborne City 1927
3 Manurewa 975
4 WDU 521
5 North Shore United 530
6 Dunedin City 453
7 Christchurch United 618
8 Hamilton 569
9 Rangers 773
10 Stop Out 464
11 Blockhouse Bay 521
12 Nelson United 548
I had some stats for the early years of the NZFC but can't find them although from memory Auckland City were best attended with around 1500 average while Southern (or Otago United as they were) struggled to get 100.
Until they expanded the NZFC the record crowd was 2500 for Canterbury United v Team Wellington - a game which was played in Nelson!