Pakistan fell 68 runs short in their 276-run chase which prompted Waqar to call for extra hard work in fielding and batting against the short ball.
“We need to improve our fielding,” said Waqar, after his team lost the fifth and final match in Abu Dhabi.
“It (fielding) is not up to the mark and must improve a lot and we also need to work hard on short balls as we have to go to Australia and both these things need to be spot on if you want to compete in the World Cup.”
Australia and New Zealand co-host the World Cup from February 14 to March 29.
Pakistan now have just two one-day internationals in New Zealand on January 31 and February 3 before the World Cup gets under way.
“I think we must keep hope, we are trying our best. I have full hope in this team for the World Cup, we will do well,” added Waqar.
The Pakistan coach said he had achieved some of his objectives from the five-game series against New Zealand.
“We did achieve what we needed but not 100 percent. Now we have a one-day cup back home and hope that things will be clearer after that,” said Waqar.
Pakistan’s problems were compounded by injury to regular captain Misbah-ul Haq who missed the last three games due to a hamstring problem.
They were already without ace spinner Saeed Ajmal who was suspended in September due to an illegal bowling action while all-rounder Mohammad Hafeez was also banned from bowling for the same reason last month.
New Zealand stand-in skipper Williamson described the series win as a “big achievement”.
“There is still time for the World Cup, it’s good it will be in our conditions and this series win is a big achievement,” said Williamson.
New Zealand’s win was more significant as they rested regular captain Brendon McCullum and spearheads Tim Southee and Trent Boult.
Williamson, whose team levelled the series 1-1 and then 2-2, said they had learned from mistakes.
“We learned from our mistakes, beat a strong Pakistan side and have shown improvement.
“In a series that went up and down, it was fantastic to learn from the Pakistan side and to win in such an evenly fought series has been great.”
Williamson termed the rotation of players as important.
“It was important to keep adapting our plans, the personnel changed quite a lot, we had a lot of injuries,” he said.
“It has been a great series, an emotional series for many reasons and both teams put on some fantastic cricket.”
New Zealand will next host Sri Lanka for two Tests and seven one-dayers- AFP