My professor said that New Zealand was essentially forced out of the Empire despite a lack of support for the idea of complete independence from the population and New Zealand politicians. Is this really the case? Were New Zealanders really forced to become independent, or am I just misinterpreting what he said.
I think your professor's language is a bit harsh, but he is trying to emphasize a point.
New Zealand (along with Canada, Australia, and a few other countries) didn't achieve independence in an abrupt manner in the same way India and the USA did. These countries made a gradual shift towards self-governance.
The biggest changes occurred after Britain passed the Statute of Westminster in 1931. Some countries (like Canada) then took responsibility for passing their own laws, and managing their external affairs. Some countries (like New Zealand) we're required to specifically adopt the statute, which New Zealand did not do until 1947.
So, it's not like New Zealand got "kicked out" in in 1931 (or 1947), but the message from mother England was clear. The large (even NZ), established dominions were ready to take the next step towards self-governance. By 1947, colony divestment was in full swing. As JBC mentions, Queen Elizabeth II is still the head of state.
It was not really forced out, but we did not really elect to leave either. The issue was that, prior to 1950, NZ had two houses of parliament - the Legislative Council (the upper house) and the House of Representatives (lower house). However, the upper house was effectively useless - its members were not elected, but instead were appointed by the Governor-General on the recommendation of the Prime-Minister. Since the GG tried to stay out of politics entirely, the LC evolved into nothing more than a rubber stamp for the PM - he chose it's members and they just signed off on Bills that got through the lower house.
As this was wasteful, the NZ government decided to abolish the LC. BUT it had been established by the New Zealand Constitution Act 1852 - an Act of the British Parliament. Since the NZ government could not override an Act of the British Parliament (or, rather, there was uncertainty if it could or not) Parliament took the "easy" option of first adopting the Treaty of Westminster, then abolishing the LC.
Becoming independent from the UK was a side effect of adopting the ToW - it wasn't that we wanted independence, but in order to do what we wanted to we (possibly) needed it.
Source: Constitutional and Administrative Law in New Zealand by P. Joseph
It wasn't really a situation of being "forced" out of the Empire, rather there was no particular drive or desire to take advantage of the Statute of Westminster.
New Zealand wasn't so much against the idea as that it felt no particular need to adopt the Statute of Westminster. New Zealand's constitutional history is really one of an odd sense of laziness and contentment with the status quo until something doesn't work - if it ain't broke, don't fix it. Partially the reason why we still have an unwritten constitution.
New Zealand wasn't the only Dominion in the situation - the Statute only automatically applied to Canada, the Irish Free State and South Africa. The section concerning the other Dominions Australia adopted it in 1942 and Newfoundland never did so.