‘A deadly new virus’
Many people believed that the second wave of the 1918 influenza pandemic arrived in New Zealand in the form of ‘a deadly new virus’ on board the RMS Niagara. The ship arrived in Auckland with cases of influenza on board just a fortnight before the second wave took hold in the city. Speculation was further fuelled by rumours that the vessel had only been cleared because two of its prominent passengers, Prime Minister William Massey and his deputy Sir Joseph Ward, had refused to be quarantined.
Influenza before 1918
The 1918 influenza pandemic was an anomaly in New Zealand and the world's experience of influenza. New Zealand experienced intermittent outbreaks of influenza throughout the 19th and early part of the 20th century. But since a serious outbreak in 1907 the rate of death had steadily declined. By 1913 other infectious diseases, such as smallpox and tuberculosis, were of greater concern to health authorities. Influenza was ‘such a familiar disease of the winter months that most people regarded it as more of a nuisance than a threat, on a par with the common cold’. Even when the disease was at its worst, during the 1890s, deaths numbered in the low hundreds – not the high thousands that the 1918 influenza pandemic would eventually bring.
The official Health Department report on the pandemic, compiled by Dr Robert Makgill in 1919, provided evidence that suggests the Niagara carried nothing more than ‘ordinary influenza’. Makgill pointed out that the ship left North America well in advance of the second wave, making it ‘hard to see where [it] could have picked up a new "killer" virus’. He also noted that ‘all three doctors who examined the passengers and crew’ declared that the type of influenza onboard was ‘no more severe than the type that already existed in the city’. It was this information, rather than pressure from politicians, that had led health authorities to clear the ship in the first place. In fact Massey and Ward took ‘no part in the quarantine decision’, insisting only that they be ‘treated like ordinary passengers’.
The timing of the Niagara's arrival may have fuelled speculation, but for Makgill it was further evidence that the ship had carried only ‘ordinary influenza’. He pointed out that if it had been carrying a deadly new virus, to which people had no immunity, ‘a rash of local epidemics’ should have appeared as people returned to their homes. They did not. Auckland's ‘explosive outburst’, and outbreaks of severe influenza in most other parts of the country, occurred ‘well outside of the 48 hour incubation period’.
Geoffrey Rice's research supports these observations. He determined that if the Niagara had been carrying a deadly new virus the peak of mortality for the whole country should have occurred on or about 7 November, not 23 November. Rice concluded that:
A balanced view of the available evidence tends to suggest that the epidemic on the Niagara was a last fling of the mild first wave of the pandemic rather than the first outburst of the deadly second wave. 
A deadly mutation or hybrid?
Evidence that the Niagara was carrying mild influenza rather than a deadly new virus may not exonerate it completely. One of the two remaining ‘plausible possibilities’ on how the second wave developed in New Zealand leaves room to blame the Niagara, as well as other vessels that arrived in the country during the same period.
The first possibility arises out of Magkill's observation that the form of flu prevalent in New Zealand in September was itself changing in October. This suggests that the second wave may simply have been a deadly mutation of the first wave. The existing virus may have lain dormant for a time, appearing again in a more virulent form in October, perhaps triggered by the poor weather the country experienced that month.
The second possibility is that the second wave was a ‘deadly hybrid’ of the variant prevalent in New Zealand in September and another variant from overseas. The overseas variant may have arrived on the Niagara, but it might equally have been aboard any of the six other ships that arrived in Auckland in October. Either way, if the overseas variant presented as mild influenza until it recombined with the New Zealand variant, the authorities would have had no cause for quarantining any ship.
Both possibilities suggest the waves were ‘virally related in some way’. This explains what Rice terms the ‘presence of pockets of very low infection and light mortality alongside places of very high mortality’. But he points out that only the latter theory explains why quarantine measures in Australia and American Samoa appear to have been effective at preventing or delaying the second wave, and why Western Samoa, which was not quarantined, was so badly affected.