The New Zealand Embassy in Brussels has not confirmed any New Zealand casualties as a result of the attacks, which took place at an airport and a metro station. There are 71 New Zealanders registered as being in Brussels, though the true number is likely to be much higher.
The Ministry of Foreign Affairs is warning New Zealanders against non-essential travel to Belgium.
"We obviously encourage them to do anything they can to be vigilant and potentially to leave," Mr Key said.
The Prime Minister also discussed the relevance of the suspected terror attacks on New Zealand. He said New Zealand spy agencies' intelligence-gathering was important to predicting incidents before they occurred.
"Because these attacks are very difficult to stop in a practical sense, when someone just walks in with suitcases and detonates a suicide bomb ... it is really important the work we do in terms of co-ordinating with other countries, in terms of sharing information, in terms of the work that our intelligence agencies do.
"Because we really have to stop these acts before they are actually carried out."
New Zealand's terror threat level is unchanged today.
"The domestic terrorism threat level in New Zealand remains low, but is under continuous review by the Combined Threat Assessment Group," a spokeswoman for SIS and GCSB Minister Chris Finlayson said.
New Zealand spy agencies are closely watching between 30 and 40 individuals who are considered a security risk. That number has not changed in the past 18 months.
International media have reported that the suspected terrorists' targeting of an airport could prompt a re-think about where people are screened within departure terminals in Europe. Like New Zealand, travellers are generally not screened before they enter a terminal.
New Zealand's airport and aviation security is currently under review.
The review was launched in late 2014 when the terror threat level in New Zealand was raised from "very low" to "low". It is ongoing, but ministers have hinted that it could lead to screening of domestic luggage, including check-in bags.
The review was looking at all aspects of aviation security, including cabin crew, baggage, cargo, catering, and ground crew.
Transport Minister Simon Bridges said this afternoon that there were no imminent threats to domestic aviation in New Zealand.
"However, we continue to monitor the security situation, both here in New Zealand and abroad," he said in a statement.
Mr Bridges did not specifically refer to the Brussels incident, but said the review of the aviation sector's security measures would take recent events into account.
"Work on the review is ongoing and is considering any current or emerging threats or system vulnerabilities, and assesses the risk levels for these."
An Auckland Airport spokesman said today that its security measures were based on advice from government agencies, and it had not been warned of an increased risk.
"We haven't received any specific advice in light of the events in Brussels, therefore our staff will continue as normal, remaining vigilant of any suspicious activity as they always are."
