[9] In World War II, it was temporarily converted into an air-raid shelter for Parnell residents, with baffle gates across the entries to protect against blasts.

[11][12] In the 1900s it became increasingly clear that the single-track tunnel, despite having a double-track line at the northern end, was creating significant 'delay, discomfort and danger'.

[14] The Minister of Railways, Joseph Ward, argued that his staff had convinced him that it was not [yet] essential to duplicate the tunnel to remediate the delays.

However, in a strange twist, he moved at the end of a meeting with a 200-strong citizens deputation that if "ten representative [business]men", selected by the Mayor of Auckland, would traverse the section with him, and then ask for the tunnel duplication, he would be willing to proceed with it.

Newspapers of the time noted that while the duplication was certainly necessary, it remained unseemly to bring about the second tunnel in this way, with a "stacked" body of men asking for it, despite the definite way in which Ward had phrased his pledge – leaving him no honourable alternative to now decline it.

Government had previously argued that up to 240 trains daily (one every 6 minutes) could be run through the single-track tunnel, and thus, the cost of £35,000-£40,000 for the duplication was not merited when other projects were of greater importance.

[15][16] Due to the lack of unanimous agreement – and because the Railways Department argued the tunnel could take twice the traffic using it at the time – duplication was again declined.

There was also a perception by the minister of the day that additional freight traffic generated by the North Auckland Line would not be worth considering as an argument for the tunnel, as it would be extremely limited.

[1] The first preparatory works occurred around April 1914, widening the approaches to the tunnel,[26] and the first air shaft connecting the two drives was pierced through in January 1915.

[1] The tunnel was immediately lauded for the improvements to rail efficiency and safety, now allowing double tracks over the whole line between Britomart and Penrose.

[29] Stormwater drainage was also improved as part of the work, with the insufficient quality of the drains (and the earlier attempts to fix them) having forced authorities to place speed restrictions on the tunnel in the past.

[33][34] In 1921, a prisoner escaped through a lavatory window while the train was making its way up the hill near the northern portal of the tunnel, and made his getaway successfully.